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In wireless networks, multirate capability will facilitate using channel resource and reducing the bit error rate effectively. Current contention-based MAC protocols, however, behave inefficiently in multirate situations, leading to the performance anomaly problem. This paper researches the throughput deterioration in multirate wireless sensor networks. Initially, this paper refers to the notion of fairness to solve the performance deterioration problem and demonstrates that the utility maximization of throughput will be achieved when each contending node receives an equal proportion of channel occupancy time. Besides, considering the intrinsic tradeoff between throughput optimization and energy consumption optimization, the paper converts the tradeoff to the regulation of single node's channel access probability, which is determined by node's data rate and energy level. Finally, based on the contention-based MAC algorithm, the paper proposes an improved MAC protocol. Simulation validates that different application demands between throughput and lifetime can be adjusted flexibly and efficiently by regulating the tradeoff factor in the algorithm.