Abstract: WSN(wireless sensor network) is formed by a large number of cheap sensors, which are communicated by an ad hoc wireless network. WSN is used to be deployed in a certain area to collect information of sensed objects. In most applications of WSN, the acquired information is useful only when the locations of sensors and objects are known. Therefore, localization is one of the most important technologies of WSN. However, the characters of WSN, such as limited wireless bandwidth and limited power and processing ability of sensor node, determine that the localization of sensor network is a challenge. In ACT (alternating combination trilateration), in order to acquire higher localization precision, all weight triangles formed by any three beacons are used to localize a sensor node. However, its calculation overhead is very high. Therefore, IACT(improved alternating combination trilateration) is proposed to improve ACT(alternating combination trilateration) by a new high-weight triangle selection method, and acquires a lower calculation overhead without affecting its location precision. However, the studies show that its calculation overhead can be reduced further. Therefore, a new high-weight triangle selection method based a cycle is proposed, and IACT is improved further by ACTBCS(alternating combination trilateration based on circle-selection) based on the circle-based selection method. Compared with IACT, ACTBCS has lower calculation overhead and similar precision of location.