Abstract: A fly dynamics model of ballistic missile to penetrate the anti-missile system in the passive ballistic curve, which was based on the increasing velocity along its original velocity direction, was established. The influences of the impulse engine work time and the impulse engine ignition time on trajectory were analyzed based on the result of trajectory simulation. With a constant total impulse, the short time-large thrust model had maximum range. When impulse engine ignited at climbing stage, the model of short time-large thrust had maximum utilization ratio of energy, while the model of long time-small thrust had maximum utilization ratio of energy at descending stage. The constraint conditions of the optimization problem of increasing velocity along its original velocity direction by impulse firing were given. The results of optimization show that the mass of impulse engine decreases 24.87%.