Abstract: A method was presented for fitting aerosol optical depth and PM_2.5 within 1 km of atmospheric boundary layer by using the ratio of Mie elastic scattering signal and Raman scattering signal of the rotational Raman-Mie lidar. It can eliminate the error brought by the geometric form factor and effects of atmospheric humidity and can obtain a high accuracy of aerosol scattering coefficients. And then an arithmetic was derived for the aerosol optical depth (AOD). Finally, the aerosol data that measured by the rotational Raman-Mie lidar system in laboratory for Beijing area in 2013, were used to get the experience formulas of AOD-PM_2.5 for winter and summer in 2013 Beijing area.