Besides delivering high data rates in a point-to-point scenario, multi-input multi-output (MIMO) antenna techniques can broadcast personalized data to multiple users in the point-to-multipoint scenario. Zero-forcing beamforming (ZFB) is a suboptimal but simple MIMO broadcast technique, which basically decouple the MIMO channel into many parallel singleinput single-output (SISO) channels. In this article, we first derive the closed-form expression for the sum rate of the ZFB MIMO broadcast system with random user selection. Secondly, under the condition with finite users, we develop a virtual user approach approximation method for estimating the sum rate of the ZFB MIMO broadcast system with exhaustive user selection. Our results indicate that the proposed analysis method can accurately estimate the optimal sum-rate throughput of ZFB.