Because of the tight coupling between the traditional network control layer and the data layer, the path allocation algorithm has poor globality and lack of real-time, and distribution of network traffic is unbalanced, which leads to network congestion. To solve the problem, according to the advantages of SDN centralized control and transparency, a Top-K routing algorithm based on Bandwidth utilization (Top-KRA-BU) is proposed. The algorithm calculates K available paths from the source nodes to the destination nodes in real time, and based on the bandwidth utilization, the K paths are evaluated, and the optimal forwarding path is selected. The experimental results show that the routing algorithm is superior to shortest path first (SPF) routing algorithm in the network bandwidth utilization.