Different to conventional wireless multi-hop networks, the router is fixed and mobility is not the main problem in wireless mesh networks (WMNs). So a new metric is required imminently to fit in with the situation. Multi-radio and multi-channel are used in WMNs to enhance network capacity. WCETT is a metric designed for multi-radio and multi-channel wireless mesh network, but the bandwidth computation required is complex. A new routing metric SMETT is proposed to avoid the complex bandwidth computation, and a routing protocol (MP-DSR) based on SMETT is presented. A transmission interference model named transmission infection factor (TIF) is designed to make the ETT closer to the real transmission. Simulation results show that the average throughput of our approach is 20% more than the original DSR and 10% more than WCETT-based DSR at least