In order to dynamically measure capturing performance of missile under HPM damage, a method called missile-target geometry count was proposed to model capturing state in different ballistic position. Based on seeker's HPM damaged model and the missile's full digital simulation system, quantitative relationship among the missile-target geometric position, the density of HPM and the seeker's capture ability was established by Monte-Carlo simulation. Theoretical analysis and simulation results show that the new method can well embody dynamics and randomness of simulations and is convenient for accumulating different experiment results under the same scene, so it is reasonable to evaluate the relationship between capturing performance and attack range.