The location algorithm having the distributed and anchor-free features is always one of the important developing directions in the node location techniques of sensor network. On the basis of analyzing a great many anchor-free location algorithm, the article finds out a distributed anchor-free location algorithm with which the location obtained is quite accurate. In order to further improve its location accuracy, the article establishes an error model of the algorithm and then proposes a method of reducing the target node positioning error by analyzing this error model. And to measure the accuracy of the estimated location of node, the article puts forward two targets, positioning level and credibility. According to the conclusion from error analysis, the article designs an optimization mechanism with the usage of the targets proposed on the basis of the original algorithm, which can filter out high-precision neighbor nodes from the neighbor table of a target node, and thereby the positioning accuracy of the target node is improved. The computer simulation shows that the availability and reliability of estimated location which are calculated by the optimized algorithm are higher than those calculated by the original algorithm, thus the feasibility of this optimization is verified.