In order to measure the modulation transfer function of the imaging sensor simply and expediently, the method with the template directly projecting on the imaging sensor has been adopted. Usually, the rectangle template is adopted, and there is a distance between the template and the photosensitive surface, which is critical in the measurement. The optical distribution on the rectangle template and the photosensitive surface with different distances are theoretically analyzed. Modulation transfer functions are derived with the optical distribution function and the sine template contact measurement respectively and compared. The experiment has been carried out, using rectangular grating with the spatial frequency 50 mm-1, and the experimental value of the modulation transfer function is 0.22, which is corrected to be 0.18. The result indicates that this modified modulation transfer function is more accurate, and the performance of the imaging sensor can be appraised more correctly.