According to the situation of coal samples, in order to achieve the full automation of coal samples, this paper proposes a locating method based on image processing for car positioning. First, this method uses of background-subtraction, image projection method, the improved Canny-edge-detection algorithm and other means to get auto maps, then combined with the size of cars to get the actual regional map. After years of sampling show that the high accuracy of the algorithm, the sampling process to resolve the issue of human factors. The method is important for the sampling the field of automated production.