Aiming at the problem that the poor recognition ability of mobile robot to AprilTag image is caused by uneven illumination intensity and overly fast movement, an improved method of AprilTag image preprocessing was proposed to improve the success rate of image recognition. Firstly, a method of tail shifting is used to change the colored image into gray image. On the basis of the above, a bilinear interpolation method is adopted to carry out downsampling on the gray image to improve the overall processing speed. The gray image is processed by histogram equalization to eliminate the influence of uneven illumination and enhance the contrast of gray image. Then, bilateral filtering was carried out for the gray image to remove the image noise, and Canny operator was used to detect the edge of gray image to improve the success rate of AprilTag image recognition and the subsequent positioning accuracy. The experiment verifies the effectiveness of the improved algorithm. The success rate of the proposed method for AprilTag image recognition is increased by more than 3% compared with the traditional method under different light conditions, and the realtime positioning error of the mobile robot is controlled within 1～2 cm, the validity and feasibility of this method are verified.