The signal-to-noise ratio(SNR) of Lidar (light detection and ranging) reflects its radiance and is one of the most important parameters for evaluation of lidar return signal quality. However, it is very difficult to estimate the value of SNR quantitatively in practical engineering. At the first, the algorithm was used to estimate lidar returns noise scale factor(NSF) and SNR. Then, the value of NSF of the Rayleigh lidar were estimated by measuring 0-40 km atmosphere day and night with 532 nm wavelength laser, which is 0.07 in daytime and 0.034 in nighttime. Finally, the SNR of the atmospheric lidar was estimated by the value of NSF in daytime. Result shows that the method which based on the NSF in nighttime of lidar to estimate the value of SNR is feasible. The evaluating of NSF and SNR of the lidar can be used as the basis for its overall design and inversion.