In the process of 3-D measurement tured light vision sensor, feature data of the of the workpiece in shape of arc with strucstructured light arc is inadequate compared with the whole circle, which results in the location of the circle centre being of low precision. Least-squares method based on radius constraint is proposed in the paper to solve the problem. Influence of the noise in sample data the on the locating of circle centre is analyzed in detail, and simulated experiments are implemented. Results of experiments show that under the circumstances that feature data of the structured light arc is inadequate compared with the whole circle, the method proposed above can improve the accuracy of circle fitting efficiently.