In this paper, we propose an asynchronous duty cycle adjustment MAC protocol, called ADCA, for the wireless sensor network (WSN). ADCA is a sleep/wakeup schedule-based protocol to reduce power consumption without lowering network throughput or lengthening transmission delay. It is asynchronous; it allows each node in the WSN to set its own sleep/wakeup schedule independently. The media access is thus staggered and collisions are reduced. According to the statuses of previous transmissions, ADCA adjusts the duty cycle length for shortening transmission delay and increasing throughput. Simulation results show that ADCA outperforms related ones in terms of energy saving, network throughput and transmission delay.