In this article, on the base of traditional impulsive factor’s limitation, the method to calculate the modified impulsive factor is proposed. The modified impulsive factor in different case is calculated by using experiment data of a underwater target bombed by underwater explosion Results show that the target’s damage state can be reflected well by the modified impulsive factor. The target’s damage distance curve is obtained by the estimated critical value of impulsive factor. Then, the measurement of probability and statistics is introduced, the target’s invalidation probability with the explosion centre changing is given by the program of Monte Carlo simulation.