Detecting light stripe center position accurately has critical consequences for the accuracy of the structured light measurement system. A robust method which extracts the light stripe center by multi step is proposed. Firstly, the noise points are moved by adaptive thresholds. Then, the initial centers of light stripes are obtained using expansion and thinning algorithm. After that, those initial centers’ normal direction can be gained with their gradient. At last, the sub-pixel center line of structured light is obtained by computing the gray-gravity along the stripe’s normal direction. The method applied to a tunnel contour deformation detection system which measuring principle is linear structured light. The test results showed that the method can accurately extract the center line.