Designing low energy consumption, high efficiency media access control (MAC) protocols are one of the most important directions in wireless sensor networks (WSN). In this paper, we proposed a new contention reserve MAC protocol, named CRMAC, under the inspiration of IEEE 802.15.4's superframe structure. CRMAC is a MAC protocol suitable for intra-cluster WSN that combines the advantages of contention and schedule-based MAC protocols. We introduce the mechanism and superframe structure of CRMAC in detail and verified the performance of this protocol through simulations. Our results show that CRMAC performs better than IEEE 802.15.4 in energy consumption, system delay and network throughput. CRMAC is especially suitable for short packet transmission under low load networks, which is the main situation in WSN.