This paper designs a forest fire detection system based on wireless sensor network. The hardware and software circuit were designed, as well as the system architecture of sensor network was described also. Meanwhile, because of the wireless sensor networks' energy constrained, energy-saving algorithm was proposed based on grid theory in this paper. The algorithm calculates the center point instead of original four surrounding nodes and the complexity is reduced. The simulation results show that the degree of error between the calculated coverage and actual coverage is less than 11%, verify the effectiveness of the algorithm.