Abstract: A preemptive scheduling strategy, IS-EDF (idle sleep-earliest deadline first) is proposed for the operation system of wireless sensor networks and implemented on TinyOS. Thus the priority of the tasks on wireless sensor nodes can be adjusted dynamically so as to ensure the high priority of important task to be executed and improve the system response characteristic of the nodes under heavy task load. When a wireless sensor node is idle, the IS-EDF can bring the node into sleep mode for energy saving. Experiment and simulation results showed that the IS-EDF scheduling strategy can effectively improve the system response characteristic of the wireless sensor nodes under heavy load at the cost of small increase in energy consumption.