Barrier coverage, which guarantees that every movement crossing a barrier of sensors will be detected, is known to be an appropriate model of coverage for moving detection and boundary guard. The related problems about barrier coverage with stationary sensors are extensively studied. When sensors are randomly deployed, we require much more sensors to achieve barrier coverage than deterministic deployment. In this paper we study barrier coverage with mobile sensors, in which the sensors can be relocated after deployment, and we are able to utilize much fewer mobile sensors than stationary sensors to achieve barrier coverage with random deployment. We study the energy-efficient relocation problem for barrier coverage, and propose a centralized barrier algorithm, which computes the relocated positions based on knowing the initial positions of all sensors. For practicability and scalability, we further design a distributed barrier algorithm based on our proposed virtual force model. We conduct extensive simulations to study the effectiveness of the proposed algorithms.