Aiming at the application background of early warning of prompt global strike weapon(PGS),the calculation method about the number of photons from a(PGS)weapon to a detector is presented.The computation techniques for the number of photo-generated electric charges that based on the quantum efficiency of detector are introduced.The effects of PGS′s radiometric and dynamic characters,the number of photo-generated electric charges and the detectable current threshold on the gain design of avalanche photodiode are discussed.It shows that a certain application is related to a optimized detector gain.It′s a from top to bottom design method.