Wireless sensor networks have been widely used in many important fields. Medium Access Control (MAC) protocols have a significant effect on the function and performance of sensor networks. At present, most MAC protocols use the same transmission power when sensor nodes send packets. However, the deployment of the sensor nodes is asymmetrical in wireless sensor networks, which will bring more energy consumption and unnecessary collisions. By jointing the MAC layer and physical layer of the senor nodes, and based on Sensor MAC (SMAC) [1] protocol, we propose a transmission power control MAC protocol in wireless sensor networks (transmission Power control in SMAC, PSMAC). The simulation results show that, compared with the SMAC protocol, our protocol has improved a lot in the delay of packets, reception rate, energy consumption and throughput of the networks.