With development of missile penetration technology, higher and higher demand is required eagerly to achieve the characteristics of attack missile early. Based on the characteristics of boost-phase of the ballistic missile, the regularity for change of attitude angle was got. With the change of attitude angle, the RCS time series of ballistic missile were calculated by the combination of physical optics (PO) and the method of equivalent current, and the result was compared with the detached missile. The result shows that the RCS time series of the boost-phase of ballistic missile embody the motion characteristics of missile, and time and velocity of detachment can be achieved preliminarily.