Abstract: In this paper,a new kind of distributed polygon surrounding detecting algorithm is put forworded,which isbased on the directed tree extension.The algorithm aimed at solving the situation of fault detection and failing to examine some parts that should have been examined in boundary detection.There is no limit to the network node density in this method.The simulation results showed that the algorithm avoided above situations effectively,the discriminant method is simple and accurate,and it improves the efficiency and precision of the WSN edge recognition greatly.