RCS of ballistic missile in the boost phase is the key information to missile early warning. In this paper, the trajectory simulation in the boost phase is built up based on the missile dynamic characteristics. A quasi-static method in conjunction with the method of moment is adopted to calculate dynamic RCS of missile in the boost phase. The missile attitude variations are obtained compared with the static RCS results of all incident angles. Finally, the original data is processed by using N point reduction method to get the joint distribution of mean value and variance of dynamic RCS, through which the trajectory characteristics are shown.