In order to measure modulation transfer function of visible light lenses conveniently and break the monopoly of foreign countries, a small and light and portable modulation transfer function tester is developed. The tester takes pinhole as target. The light from pinhole is collimated by collimator, and then passes through the lens under test, and forms the pinhole′s image on the image plane of the lens under test. This image is magnified by microscope and received by charge coupled device(CCD) and given to computer. The computer processes the image and calculates the modulation transfer function. The image processing procedure is introduced. The algorithm of modulation transfer function is provided in detail. The influence of components of the test system is analyzed and corrected. The method of restraining error and noise is proposed. This tester′ s results are compared with the theoretical values. It indicates that the absolute accuracy is 0.018, which satisfies the demand of 0.03 and achieves the research purpose.