Traditional methods regard coverage area of radar network as the union of every radar’s coverage area. Aiming at this issue, the relationship among radar detection range, radar cross section, signal-to-noise ratio, detection probability and false alarm probability is analyzed. Detection probability model for single radar is established. Calculation method of detection probability for radar network is also researched. Coverage area of radar network can be obtained according to the detection probability. Simulation results show coverage area of radar network is not simply the union of every radar’s coverage area and it is decided by the detection probability. Research of this paper provides a theoretical base of detecting, tracking and placement for radar network.