This paper analyzed the impact of the atmosphere back-scattering on the active detecting system and built a corresponding physical model based on geometrical optics. A simulated calculation of several related factors was implemented and then the illation of the interfering ability of the back-scattering was validated by a long-distance detecting experiment outdoor. An appropriate result with the illation was achieved which can be helped for the design of system involving the transmitting of the laser in the atmosphere such as laser active detecting, range measurement with laser and so on.