Abstract: This paper presents a space-time adaptive processing(STAP) method for the applications in active sonar.In order to address the problem about the performance of optimal space-time filter decreases with limited number of space-time data snapshots,by utilizing the comparability of the expression about Capon spectral estimator and optimal space-time filter,the Ridge Regression diagonal loading method is proposed for STAP applications.The simulation shows,the new diagonal loading method improves the performance of SMI-based space-time filter,and is able to suppress the surface reverberation and interference effectively.