Monopulse angle estimation algorithm based on adaptive sidelobe canceller is analyzed in this paper. Because of the mismatch problem, interested signal can not be cancelled completely by blocking matrix, which results in the leakage of interested signal in auxiliary channel. Performance of monopulse angle estimation also degrades as a result. To solve this problem, an improved method is proposed. Orthogonality relationship between signal space and interference space is used in this method to reduce the effect of the leakage of interested signal in auxiliary channel. Theoretical and simulation results are given to prove the effectiveness and superiority of this method.