Logarithmic ratio k of aerosol backscatter-to-extinction is usually as an assumed value in solving lidar equation. The relationship expression among value k, wavelength exponent, measurement wavelength and aerosol extinction coefficient is derived. A new method to determine value k is put forward. The value of k under different weather conditions is initially analyzed by using the data of 532 nm Rayleigh-Raman-Mie lidar and micro pulse lidar and combining CE-318 sun-photometer data. To verify this method, the visibility is brought to estimate the actual condition of aerosol extinction coefficient. It shows that under better air condition, aerosol extinction coefficient is closer to the result estimated by visibility only in k=1.0, and under foggy weather, when the value of k calculated by this method changes from 0.7 to 0.9, the relative error between aerosol extinction coefficient and result estimated by visibility reaches the minimum. It shows that the calculating value of k using this method has certain reliability.