The Chang＇ E lunar observation laser altimeter was designed to provide precise altimeter data to accurately map the lunar surface by measuring the time interval between the emission and receiving signal. The relationships of the background radiant power, the multiplication factor, and the optimum signal-to-noise ratio of the receiving system were analyzed. The signal-to-noise ratio experiment for different background radiant power was conducted by using the integrating sphere. The influences of background radiant power and APD mutipliction factor on the signal-to-noise ratio of the receiving system were validated. The integrating sphere＇ s simulation test shows that there are variant optimum multiplication factors to achieve the maximum signal-to-noise ratio under variant background radiant power conditions.