The software overhead in IN (interconnection network) communication has become currently the bottleneck of cluster system performance. In this paper, a method is proposed to reduce the communication software overhead by providing a fast message passing mechanism at user level——ULFM (user level fast message), that has been implemented in the IN TH-GBNet of cluster system. In ULFM, a new type of network access interface is provided to form a communication buffer centric application access to the cluster system IN. Several techniques, such as user level network access, zero copy message passing and simplified communication protocol, are integrated together for ULFM, that can effectively reduce the communication software overhead and achieve high communication performance with low latency and high bandwidth for application programs. Experimental results show that the latency at application level between different nodes for 16-byte message can be reduced to only 6μs, and the bandwidth can reach to 40MB/s for 2K byte length message.