To solve the secure localization problems, a number of secure localization schemes have been developed at present. However, most of these techniques cannot survive collusion attacks where a majority of malicious nodes launch colluding attacks. In this paper, we introduce a new collusion attack model called Collaborative Collusion Attack Model (CCAM) and propose a novel scheme called Two-Step Format Detection (TSFD) that is well suited to WSN which is a resource constrained environment. The TSFD has reasonable and acceptable communication cost and algorithm complexity. Through simulations, we compare the performance of TSFD with other secure localization schemes and show that TSFD has more efficient and resilient performance.