The complexity of direction-of-arrival (DOA) estimation method for massive multiple-input multiple-output (MIMO) systems is critical and extremely affects its application. To reduce the computational complexity, we propose a new low-complexity DOA estimation method based on the beamspace transformation and the improvements of propagator method named as low-complexity beamspace-based improved propagator method (LBPM). Specifically, the received signal vectors in the antenna-element space are transformed into the beamspace by employing beamforming vectors, which simplifies the array manifold so that the total dimensions of the received signal vectors are significantly reduced. Then, to further decrease the computational complexity, the linear operation of propagator method is utilized to replace the eigenvalue decomposition (EVD) of the covariance matrix. Besides, the transformed data is conjugated and rearranged to improve the precision of the proposed method. Numerical simulation results indicate that the proposed method enjoys better DOA estimation performance than that of the conventional methods in massive MIMO systems.