Because the measure angle error of ARM greatly affects the hitting precision and killing probability, for the ground -to-air missile there exist the nearest safe distance and time of closing the radar when firing the ARM attacker. Based on this, the paper is the first to put forward the concept of the best - firing region when the ground-to-air missile firing the ARM attacker. The sectional views of the vertical and horizontal best-firing regions under two different speeds of ARM assaulting are obtained through the simulation model.