This paper presents an enhanced positioning probability algorithm. This algorithm is a completely improved algorithm for locating mobile nodes in defective environment of trilateration location system. The positioning node can find its location with just two reference nodes and their neighbor information. Using numerical analysis model and ad hoc network model, this paper analyzes the effects of positioning probability enhancement and control messages overhead. And this algorithm is also implemented on Linux systems and we test the system at playground. In the pure lateration location system, when a locating node has just two reference nodes, the probability of its positioning is zero but our system is about 80% positioning rate with the average control messages less than 0.5 at 100 reference nodes.