To obtain nanometer thin film thickness fastly and accurately, a formula of linear fitting method based on the periodic Kiessig fringes for thickness determination is applied, and a series of SiO2 nanometer films on Si substrate with the film thickness ranging from 10 to 120 nm have been calculated with the formula. These samples are prepared with thermal atomic layer deposition (T-ALD) process and film thickness is measured with grazing incidence X-ray reflection (GIXRR) technique, in addition, the linear fitting procedure and several influencing factors among it are studied, all of the work is based on the reflectivity curve from GIXRR experiment. While at the same time, another fitting method based on a soft named Global Fit2.0 is brought into this study to compare the two obtained thicknesses from two kinds of analysis methods. In the end a novel method for film thickness determination-empirical curve is presented. The results show that: during the linear fitting process, the peak position series have a main effect on thickness determination, thickness will increase when the peak position adds up; Besides, any peak’s corresponding reflection angle also has a significant effect on the thickness determination, it is expressed in the form of interference fringe period, thickness will decrease while the interference fringe period increases, however, the errors from either peak series or fringe period can be further weakened with trial and error method, calibration procedure of critical angle and interference fringe period individually. Choosing the same sample with random thickness, no matter using the linear fitting and soft fitting method, the two gained film thicknesses are consistent and the thickness deviation is less than 0.1 nm, which illustrates the accuracy of linear fitting method for thickness determination. An empirical relationship between film thickness and interference fringe period is then put forward on the foundation of the accurate thickness determination, according this curve, the target film thickness is directly got by putting an interference fringe period in the empirical curve. This novel method not only avoids the messy procedure of choosing peak position series or their corresponding angles during linear fitting process, but also avoids the complex task of building a correct structure for soft fitting process; it is of great significance in confirming thin film thickness with quick speed and high accuracy.