In this study, an improved slanted edge method was proposed. It had a high accuracy for measuring the Modulation Transfer Function (MTF) of electro-optical imaging system. We put forward an automatic determination of the high and low threshold modified Canny operator to detect the edge points, and the edge slant angle is obtained by linear fitting of the edge points. In the process of fitting the edge spread function (ESF) by Fermi function, we put forward a modified method to reduce the influence of noise. It was composed of averaging in adjacent region and setting the tolerance of error. The method in this paper is proved effectively by the experimental results, and analyzing the influence of the change of the edge slant angle and noise on the MTF measurement accuracy. The experimental results show that the measurement accuracy of the modified slanted edge method is 0.004 9, and the repeat accuracy is 0.003 1, which is higher than the traditional methods. It proves that the modified slanted edge method has high measurement and repeat accuracy, and also has a better immunity to noise and edge angle.