The advantage of target measurement with area array image sensor for small arms is analyzed. The principle of the intersection measuring is presented. Then the mathematic model is constructed. There is not iterative calculation but only instead calculation in the model .So the model is simple. At last the focal length and image subdivision are computed and analyzed. Through experiment and synthesis analysis for the effect of the mixed focal length, image subdivision and the initial angle error, the relative error is up to 0.5% by statistic calculation. The precision can meet the 1% relative error demand of the system.