This paper expatiates on the basic characteristic of OSPF (open shortest path first), and analyzes the local congestion issue of network owing to traffic aggregation. We improved OSPF and brought forward cost adaptive OSPF (CA-OSPF). CA-OSPF is a compatible expand of OSPF, a router running CA-OSPF may dynamically adjust its interface's cost according to the utilization ratio of the interface's bandwidth, as a result, the network's load is balanced, the network's QoS and resource utilization ratio are improved. This paper expatiates on the basic idea of CA-OSPF, analyzes the simulation result and appraises the performance of CAOSPF.