In this paper, an improved adaptive monopulse scheme based on modified projection approximation subspace tracking deflation (MPASTd) approach is proposed. Owing to the fast convergence speed of MPASTd for recursively estimating the interference subspace, the proposed algorithm can obtain the formula for adaptive monopulse ratio calculation by a subspace projection. Therefore, it is able to accurately estimate the source location using less training data and reduce the computational complexity significantly. The effectiveness and fast convergence speed of the proposed method is verified by simulation results.The rest of this paper is organized as follows. Section 2 summarizes the principle of adaptive monopulse algorithm. The proposed MPASTd-based subspace method for adaptive monopulse is presented in Section 3. First, we briefly introduce the eigencanceler, and give the weight vector based on eigen-decomposition (ED). Then the improved adaptive monopulse scheme based on MPASTd is introduced in detail. In Section 4, the computer simulation results are given to demonstrate that the proposed approach outperforms the original adaptive monopulse while reducing the computational complexity and the convergence time. A brief conclusion is given in Section 5.