Compared with interferometry and shadowgraph, Background Oriented Schlieren(BOS) has significant advantages of a large field of view and low cost as a new flow field measurement technology. Developed on the basis of traditional schlieren technology and modern image processing technology, BOS has developed rapidly and has been widely used in a variety of flow field measurements and displays since its invention in 1998, such as combustion field diagnosis, temperature field visualization and density field reconstruction. Consists of a random background, a flow field and an image acquisition and processing system, the device of BOS is simple and can be used in a variety of scenarios, but it also has the limitation of over-reliance on background and algorithms because the core of this technology lies in the use of image processing algorithms, the displacement of pixels are calculated by comparing the original background without the flow field and the disturbing background after passing through the flow field. Furthermore, the pixel offset of the image represents the refractive index gradient of the flow field. In theory, the refractive index can be reconstructed from the gradient with the phase reconstruction algorithm, which is of great significance in the study of flow field structure and fluid characteristics. Firstly, the influence of five different dense optical flow algorithms such as Farneback, Sparsetodense, Deepflow, Pcaflow and Dualtvl1 and the background under different brightness, contrast and correlation on the displacement detection accuracy is analyzed by bilinear interpolation. The research found the following rules: 1) the Farneback algorithm takes the least time and has the best stability as well as the highest accuracy; 2) among the optional parameters of Farneback, the average window size ought to be determined according to the pixel displacement, and the number of iterations should not be too much to avoid time-consuming; 3) the detection accuracy of the optical flow algorithm is negatively correlated with the image correlation, positively correlated with the image contrast, and almost independent of the brightness; 4) under the appropriate algorithm and background, the displacement detection accuracy of the BOS system can be up to 1/400 subpixel. Secondly, after obtaining the pixel offset through the optical flow algorithm, according to the quantitative relationship between the background image displacement and the flow field wavefront gradient, the flow field wavefront can be reconstructed by the Fourier transform method and the iterative method respectively. The Antisymmetric Partial Derivative Integral (ASDI) method is a type of Fourier transform. A least squares error model between the measured slope and the actual slope of the wavefront is constructed firstly, and the integral operation in the space domain is mapped to a linear combination of Fourier primary functions in the frequency domain,and the original gradient matrix is filled and expanded to twice, and then the optimal solution is obtained. The main idea of the Guass-Seidl (GS) iteration method is to generate the current latest refractive index value by iteration, and use the latest value to calculate the next step of the latest parameter, and bring it into the calculation of the refractive index of the surrounding points, and so on until the convergence condition is reached. After numerical simulation, we draw the following conclusions: 1) the ASDI is faster but has lower accuracy, which can be applied to real-time processing; 2) the GS iterative method is slower but more accurate and can be used for post-processing. In practical application, we can choose the appropriate algorithm according to our needs. Finally, a set of high-precision background oriented schlieren flow field detection system was built. In the laboratory, the laser speckle pattern is used as the background, and the temperature pressure gradient is generated by the alcohol lamp combustion as the flow field. After processing the background pictures before and after the alcohol combustion with the algorithm, the obvious fluctuation of the optical path difference above the flame can be clearly seen. In the natural environment, with the forest as the background and the huge sound pressure gradient generated by the gun barrel as the flow field, images were continuously collected, and the propagating sound waves were seen after algorithm processing. The above two experiments prove that under different conditions, the BOS device can achieve both qualitative observation and quantitative measurement. The high-precision background schlieren system is simple in device and mature in the algorithm. In addition to the temperature pressure field and sound pressure field shown in this paper, the refractive index obtained by BOS can also be used for temperature and density calculation of thermal field or density field. The extracted wavefront information also helps to further study the flow field structure, and has a wide range of application prospects in scientific research such as adaptive optics and target detection, as well as in industrial fields such as precision instrument detection, engine performance improvement, aerodynamic shape optimization and so on.