The coexistence problem between M-WiMAX and WCDMA systems is discussed. To reduce the interference from M-WiMAX system to WCDMA system, the transmit beamforming technique is applied. At the same time, an adjacent channel interference model is proposed to analyze the gain induced by the application of transmit beamforming technique. In addtion, a novel sub-channel allocation algorithm is proposed to increase the throughput of M-WiMAX system. To verify the proposed algorithms, a system-level simulation platform is built up and evaluated by the Monte Carlo method.