Intrusion detection is a significant application in wireless sensor networks (WSNs). S. Kumar et al have introduced the concept of barrier coverage, which deploys sensors in a narrow belt region to guarantee that any intrusion across the region is to be detected. However, the practical issues have not been investigated such as scheduling sensors energy-efficiently while guaranteeing the detection probability of any intrusion across the region based on probabilistic sensing model, which is a more realistic sensing model. Besides, the intruders may be humans, animals, fighter planes or other things, which obviously have diverse moving speeds. In this paper, we analyze the detection probability of arbitrary path across the barrier of sensors theoretically and take the maximum speed of possible intruders into consideration since the sensor networks are designed for different intruders in different scenarios. Based on the theoretical analysis of detection probability, we formulate a Minimum Weight ∈-Barrier Problem about how to schedule sensors energy-efficiently. We show the problem NP-hard and propose a bounded approximation algorithm, called Minimum Weight Barrier Algorithm (MWBA) to schedule the activation of sensors. To evaluate our design, we analyze the performance of MWBA theoretically and also perform extensive simulations to demonstrate the effectiveness of our proposed algorithm.