The traditional optical lens detection is mainly based on echo intensity (cat-eye effect). However, there are obvious diffraction phenomena in reflection echoes for micro-lens. Thus, it is important to investigate the diaphragm diffraction effect of micro-lens. Based on the theory of angular spectral diffraction, we study the spatial modulation characteristics of micro-lens for irradiated laser, and deduce the equation for optical field distribution of echo signal. The influences of the detection distance, incident angle, and diaphragm diameter on the diffraction optical field distribution are simulated, as well as the maximum detectable incident angle under the condition of different diaphragm diameters. Moreover, the diffraction phenomena are detected using the constructed active laser detection system. The results indicate that the diffraction patterns observed from the experiment match well with the simulated patterns. The theory, developed in this article, is proved to be qualified to predict diffraction profiles of micro-lens' echo signal to an extraordinary degree.The theory can accurately predict diffraction phenomena in echoes for micro-lens.