In order to achieve high-precision measurement of surface defects of precision optical elements, a surface defect detection method of optical elements based on multispectral image fusion is proposed. By using light sources of different wavelengths incident on the optical surface, the detection results of optical surface defects illuminated by single wavelength light sources of 450nm, 532nm and 650nm are obtained in the micro dark field imaging system. After the collected images are fused by three methods: weighted average method, Laplace pyramid transform method and wavelet transform method, then image processing is performed to obtain the defect size information. By comparing the experimental results, compared with the defect detection results of single wavelength original image, the detection results of multispectral fusion image are more accurate, and by comparing the results of three fusion methods, the Laplace pyramid transform fusion has the best effect.