Abstract: The coverage problem in wireless sensor networks is studied,which is very important to network life time,sensor deployment strategy,communication protocol,networking,etc.A modified sensor model,which keeps the physical meaning of the old model and describes more general cases,is used to analyze the relationship between target speed and detection probability.The definitions of two critical speeds of the target(undetectable speed and partially detectable speed) are given,and the functional relation between these two speeds and sensor parameters are presented.When the target speed is greater than the undetectable speed,the sensor can't detect the target,and when the target speed is greater than the partially detectable speed and moves backward to the sensor,the sensor can't detect the target.The simulation results demonstrate the validity of the theoretical analysis.