Beamforming is one of the most important transmission technologies to improve the quality of communication in cellular network systems. However, in massive multiple-input-multiple-output (MIMO) systems, conventional optimization-based iterative beamforming algorithms often suffer from high computational complexity and thus are not suitable for practical applications. This paper focuses on low complexity beamforming technique for multi-user massive MIMO systems. Specifically, a block-diagonal zero-forcing (BD-ZF) beamforming algorithm is proposed for achieving weighted sum-rate maximization. We show that the BD-ZF beamforming problems can be globally solved using water-filling algorithms. Extensive simulations demonstrate that the proposed BD-ZF beamforming method can offer better performance than the state-of-art low complexity beamforming technique ZF (even could sometimes coincide with the performance of the popular WMMSE algorithm) but with only an extra little bit computational overhead.