The Radar Cross Section (RCS) model based on precession of ballistic missiles in midcourse phase is built up. A new method is proposed to extract the precession period by combining Auto Correlation (AUTOC) with Average Magnitude Difference Function (AMDF). After the precession period is obtained, match searching method is adopted to extract precession angle. Then inertial ratio can be obtained based on the extracted precession period and precession angle, which is an effective feature for making identification to the ballistic missile. From the view of parameter estimation, the Cramer-Rao Bound (CRB) of inertial ratio is analyzed, and the mean square error of inertial ratio is compared with the CRB, which verifies the validity of the inertial ratio extracting method.