The ballistic simulation model is established for a rocket. A lot of simulation target practice of the rocket is done with Monte-Carlo method to the rocket under none-control condition. The allowable deviations of motion parameters of the rocket at a series of time-points in the power-flight phase are statistically counted from all success trajectories. By setting up motion parameters at above mentioned time-points as initial conditions respectively, the dispersion of falling points to the ground is analysed by simulation regarding to the influence of motion parameter deviations and the other random deviations. The suitable simple control terminal point and its deviations of motion parameters are found out, which provide useful information for designing simple control system of the rocket.