Based on the data of atmospheric extinction coefficient detected by Mie scattering lidar from June 2013 to May 2015 of Chengdu, it was found that there was a S-shaped transitional zone between aerosol extinction and molecular extinction above mixing layer, and sigmoid function was used to simulate this vertical distribution. By obtaining the heights of the upper and lower maximum curvature points of the function, a sigmoid algorithm for calculating the transition zone of particle boundary layer based on Mie scattering lidar was proposed. The results showed that, as to the atmospheric extinction coefficient in the transition zone and its vicinity, simulated profiles and measured ones maintained a significant correlation, and the correlation coefficients of the two in spring, summer, autumn and winter were 0.9971±0.0052, 0.9935±0.0167, 0.9979±0.0038 and 0.99895±0.0021 respectively (Passing the significance test of 0.05). Further researches indicated that there existed a good correspondence between the transition zone of particle boundary layer calculated by sigmoid algorithm and the inversion layer identified by sounding data in Wenjiang station.