To meet the requirements of quick and violent attack target, establish the new fire control arithmetic of multiple rounds simultaneous impact (MRSI) by using 1 dimensional trajectory correction projectile drag ring to adjust the range and flying time. The shortest fly time principle was selected, the flying time and firing angle of the last projectile was calculated at first, and then the flying time and firing angle of reciprocal next projectile were calculated. The simulation results show that e new arithmetic is feasible and it is valuable to improve the ability of gun to attack quickly.