The spatial resolution of traditional optical microscopy imaging technology is limited by the optical diffraction limit, which can only reach the order of half wavelength of the incident wavelength, which greatly limits the application scope of optical microscopy technology. Among the mainstream optical super-resolution microscopy techniques, Structured Illumination Microscopy (SIM) is an attractive choice for fast super-resolution microscopy due to its fast imaging speed, low phototoxicity, and no additional complex sample preparation process. SIM technology applies periodic sinusoidal fringe structured light with a spatial frequency close to the diffraction limit to the sample. The high-frequency information that could not pass through the sample is now converted into a low-frequency "Moire fringe", which couples the high-frequency information of the sample beyond the diffraction limit to the imaging. The frequency region detectable by the system can theoretically double the lateral resolution of the microscope. The SIM image reconstruction algorithm is the key to determine the final super-resolution image quality. Traditional frequency-domain image reconstruction algorithms need to estimate the initial phase, spatial frequency and other parameters of the structured light field. It is also necessary to perform multiple Fourier transforms between the spatial domain and the frequency domain. The operation speed is slow affecting its application in real-time dynamic imaging and other fields. The proposed spatial-domain super-resolution imaging algorithms are currently limited to solving the 2π/3 phase shift for structured light super-resolution reconstruction. In the traditional structured illumination microscopy based on the projection of the digital micromirror device, the structured fringes with a period of 4 pixels need to adopt a phase shift interval of π/2. The spatial domain reconstruction algorithm under this application has not been reported yet. Here, a super-resolution image reconstruction for SIM in the Spatial domain is proposed. The algorithm is called differential SIM, or DIFF-SIM for short. First, three structured light illumination images with phases 0, π/2, and π are obtained. The two adjacent original images are subtracted to achieve the elimination of background interference, at which point we obtain two new expressions. Simplify the subtracted expression using the two-angle sum-difference formula of trigonometric functions. Then, we construct a new complex function, and take the simplified two expressions as the complex real part and the complex imaginary part respectively. Next, Euler's formula is used to simplify the complex number to the e-exponential function. Using the convolution formula, the function is transformed, and then the Fourier transform frequency shift characteristics are used to further simplify the expression. Finally, by taking the modulo of the result, the frequency domain spectrum of the system is expanded to obtain a super-resolution reconstructed image. Theoretically, when the structured light frequency is equal to the cutoff frequency of the system, the algorithm can double the lateral resolution of conventional microscopy systems. It is verified by simulation that the resolution of DIFF-SIM is nearly 1 time higher than that of wide-field imaging. Meanwhile, the super-resolution reconstruction effect of DIFF-SIM and FairSIM, the frequency domain reconstruction method of SIM, is the same. In a projected structured illumination microscope based on digital micromirror devices, spatial domain super-resolution reconstruction experiments were performed on fluorescent microspheres and bovine pulmonary artery endothelial cells. The system employs computer-controlled digital micromirror devices for fast fringe generation and uses multicolor light-emitting diodes for illumination. Firstly, the system resolution analysis experiment was carried out, and the system Point Spread Function (PSF) obtained by the fluorescent microspheres proved that the algorithm could expand the system resolution. The experimental results of the resolution expansion are close to the theoretical value. Then, super-resolution reconstruction of bovine pulmonary artery endothelial cells was performed, and the experimental results were compared using the frequency domain method FairSIM. The experimental results show that the DIFF-SIM algorithm can obtain super-resolution image reconstruction effects similar to FairSIM. In the defocus experimental verification of the groove of the coin, it is proven that the algorithm can eliminate the interference of the defocused background focal plane information similar to laser scanning confocal microscopy (LSCM). To evaluate the efficiency of the DIFF-SIM algorithm, a comparison of the FairSIM and DIFF-SIM algorithms is performed on the same computer, and the execution speed of DIFF-SIM is approximately 5 times faster than that of FairSIM. This study is conducive to expanding the application range of the SIM method, helping to take advantage of the technical advantages of SIM with low light dose and low phototoxicity, and has good application potential in dynamic imaging of living cells and long-term monitoring.