The classic APIT(approximate Point-in-Triangulation test) algorithm has been utilized in node localization in wireless sensor networks for major applications, however, it is easily fall into nodes misjudgments and node failure as its accuracy can not be guaranteed in low node density scenarios. To solve this problem, this paper proposes an improved node localization algorithm UNION-APICT(union approximate Point-in-Circumcircle test) which focuses on improving the localization accuracy by combining the connected ranging method, the RSSI ranging technology, the centroid localization and APICT together, which can solve the unknown node localization problem effectively. The simulation experiment results demonstrate that the UNION-APICT can achieve the lower average location error as much as 10% to 25% than APICT scheme. And when the communication radius R and the maximum detection range Rmax increasing, the node localization error decreasing accordingly. Compared with the APIT and APICT localization algorithms, the new UNION-APICT can get performance improvement significantly both in anchor nodes density, the coverage, and localization accuracy.