In order to realize anti-alteration function for License Plate Recognition System (LPRS), a uniform-field imaging system is designed and a corresponding anti-alteration algorithm is proposed. First, reflection characteristics of license plate and typical alteration material are measured. As a result, the two characteristics in near-infrared range fluctuate moderately and the former is notably lower than the latter. Then the uniform-field imaging system for visible-light and near-infrared is designed to capture the difference above effectively. Finally, the anti-alteration algorithm, composed of license plate location, character matching segmentation and alteration recognition, is introduced. Experimental results have indicated that visible-light and near-infrared images can be acquired stably by the proposed system under the condition of natural illumination and there are discriminable gray differences between license plate and alteration material in near-infrared images; and that success rate and average executive time of the algorithm are 86.5% and 157ms respectively.