Error control is a basic technology to ensure multicast reliability. Reasonable buffer management can improve error control efficiency. This paper has put forward an improved reliable multicast protocol (IRMP) on the basis of randomized reliable multicast protocol (RRMP). IRMP used a short buffer to meet local recovery requirement, and used a long buffer to meet remote recovery requirement. In local recovery, the retransmission request was sent by multicast, and the randomized back-off algorithm was used to reduce repeat message. In remote recovery, a designated repair node (DRN) was responsible for the repair. In order to avoid the quit or default of DRN, this paper has put forward a method for DRN transfer. Simulation results for IMRP and RRMP in NS2 have shown that IMRP has good ability to reduce the buffer requirements and recovery latency, and also improves its scalability.