In order to better realize non-invasive, real-time and dynamic skin tomographic imaging and improve the imaging signal-to-noise ratio (SNR), the performance of the reflective laser confocal system was evaluated comprehensively. Structure function as variable metric for wave front was introduced, and the basic mathematical properties was analyzed. The wave aberration of different scales in the structure function was analyzed emphatically. After that, the numerical simulation for low order aberration presented by Zernike polynomials was investigated. The ability of structure function to characterize low order aberrations was verified. Because confocal microscopy was required to pass through the skin, it was important to think about the effects of human tissue in the pre-test process and analyze its effects on the system′s SNR; Finally, the wavefront of the actual skin lens system was analyzed, and a SNR of 95.7 dB was obtained after system calibration, and the correctness and feasibility of the method were verified. By means of the variable scale evaluation method, the non-invasive imaging system can be evaluated more comprehensively and it can effectively guide the detection and adjustment of similar equipment.