Aim at relative coordinate conversion requirement in the conventional weapon system range test, a coordinate conversion method of relative position relation is put forward between two positions. Firstly, this method built mathematical model that converted geographical coordinate to relative angle and distance between two positions, then based on the model, analyzed weapon system geographical coordinate and relative angle distance between target and weapon system, geographical coordinate mathematical model of target was solved by iterative method. Used C++ builder compile program to realize automatic and batch conversion. At last data conversion precision and validity was validated by geographical measure harvest position. The conclusion indicates: this method calculation precision is high, and calculation speed is rapid, it can resolve the relative coordinate conversion problem in range test.