Abstract: In order to analyze the back signals and the signal-noise-ratio for the detection of He density in thermosphere by means of resonant lidar,the effective backscattering cross-section of about 10-17m2 was obtained after computer simulation.The size of the back signal was analyzed through numerical simulation with neglecting and considering the background signal respectively.Results showed that the signal-noise-ratio for current system was low,and that the range of the resolving power Δz should be decreased and the integral time τ should be extended in order to improve the signal-noise-ratio.There results are helpful for the detection of He density in thermosphere by means of resonant lidar.