Based on the weighted least square method, the mathematics model of the location algorithm about infrared multi-sites passive system is established,meanwhile the equation is offered and the realization of the algorithm is discussed. Because of the weighted least square method, much detected information can be combined efficiently,and the positional accuracy and reliability of system can be raised. The simulation result show that a large number of sites relates to a proper relative position to the sites,and a short distance between the target and sites corresponds with a accurate position.