In this letter, we investigate the integration of multiuser multiple-input multiple-output (MU-MIMO) technologies with unmanned aerial vehicle (UAV) communications. We jointly optimize the placement and the MU-MIMO beamforming matrix of the UAV for maximization of the sum rate of all users. Due to the complexity of MIMO rate expression and matrix inversion operation, we propose a generalized transformation method to decouple the variables and find an equivalent tractable function of the MIMO rate for the original problem based on weighted minimum mean square error (WMMSE) methods. To solve the problem, an alternating algorithm combined with the Lagrange dual method is proposed where the semi-closed-form solutions for UAV placement and beamforming scheme are obtained at each step. Simulation results verify the effectiveness of the proposed method and show that the MU-MIMO technique can significantly improve performance in UAV networks.