In the three-dimensional （3D） measurement of large-scale casting slabs using a multi-line structured light system， the accuracy of extracting the central stripe of the structured light is an important factor affecting the final 3D measurement accuracy. In this study， a multi-line structured light center stripe extraction method based on the optimization of the traditional gray gravity method is proposed. For linear structured light stripes projected onto the slab surface whose laser cross-section does not fit a Gaussian distribution， firstly， the environmental noise is removed by analyzing the image background difference. The boundary of linear structured light is determined according to the change in grayscale value between the linear structured light stripe and the background， and the region of interest of structured light is extracted. Then， the adaptive gray threshold of the gray gravity method is calculated according to the gray integral proportion of the light stripe in the gradient direction. The gray gravity method is used to process the region of interest and extract the approximate center point of the light stripe. Finally， the sub-pixel center coordinates of the light stripe are obtained by optimizing the extraction combined with the gravity of the roughly located center point in the neighborhood with a radius of 5 pixels. The experimental results show that this method has high precision， high speed， and strong robustness in extracting the center of the laser stripe on the surface of a casting slab with uneven reflection characteristics. The standard deviation of the 3D measurement results of the final casting slab edge trajectory points is within 2 mm.