In order to resolve antiaircraft artillery system encounter resolving simply, based on principle of target and artillery encounter in antiaircraft artillery fire control system and the physical meaning of eigen value in fast iterative method, calculate eigenvalue by using the method which average velocity method use to calculate artillery terminal velocity in approximate calculation encounter point. The method is simple, and can reduce the calculation amount. The result of analysis indicates that the iterative convergence velocity of this method is equal to its definition formula result.