In order to solve the problem imprecise alignment and uneven grinding thickness in the automatic grinding system of the locomotive, a algorithm based on the laser scanning technology for the center axis of the grinding tool in the normal direction of the grinding point is proposed in this paper. First, the normal vector of the three-dimensional model of the workpiece to be polished obtained by scanning by the line laser sensor is calculated, and the 6 methods of obtaining normal is compared to obtain the optimal algorithm to calculate the normal coordinates of each point on the model. Then, through hand-eye calibration, the normal coordinates and the center axis coordinates of the grinding tool can be converted to each other, and the two can achieve precise alignment by using offline programming. Finally, a grinding model is established to obtain the ideal grinding thickness by calculation, and the grinding thickness values of the precise and imprecise alignment are measured. Experimental results show that, compared with imprecise alignment algorithm, the algorithm can well solve the problem of uneven grinding thickness and achieve the grinding effect consistent with the ideal grinding.