Because of high accuracy and wide range, the advanced optoelectronic imaging deflection system has become the important device of the Bridge Structural Health Monitoring System (BSHMS), and has been applied to several bridges. However, the recognition accuracy of the optoelectronic imaging deflection system has greatly descended for fog and sunshine in practice. By analyzing the principle of image-based deflection measurement, a conclusion was drawn that small error in light source target in image recognition caused larger error in deflection because of amplification of optical measurement system. A novel method based on projection and differential is proposed for recognition of light source target. Compared with conventional method (its Mean-Square Error (MSE) is 16.534 08 and 23.018 13), the proposed approach (its MSE is 1.632 231 and 2.182 247) is more accurate and accords with practice. Experiment results verify the effectiveness of the designed method and theoretical discussions.