This paper mainly solves the problem of accurate positioning of reference points in deflection measurement of concrete structures. The fitting algorithm of Gaussian spot is derived. The existence of the parameter Krame-Rowe lower bound is proved, and the corresponding parameters are optimized by the Levenberg-Marquardt method. The experimental results show that the root mean square error attenuated by the center extracted by the Gaussian fitting spot localization algorithm based on nonlinear parameter optimization is 0 when the signal-to-noise ratio is 40 dB. As the contrast decreases, the extraction accuracy of the three methods deteriorates. The Gaussian fitted spot localization algorithm based on nonlinear parameter optimization is superior to other algorithms before the contrast drops to 25% of the original image, but this advantage disappears when the contrast continues to decrease. It shows that in the case of good contrast, it can not only ensure accuracy but also improve robustness.