The principles of optical Modulation Transfer Function(MTF) measurement by the edge method for an optical system were researched and an improved inclined edge method for the MTF measurement of an infrared imaging system was proposed. As the edge angle measurement error and noise will cause the MTF measurement error of the infrared system, the Canny operator, line fitting and Edge Spread Function (ESF) reconstruction row number changes were combined to improve the measuring accuracy of the MTF, meanwhile, an effective denoise algorithm for the ESF and the Line Spread Function (LSF)was used to reduce the effect of noise on the MTF measuring accuracy.With the methods mentioned above, the measurement errors of MTF were reduced systematically. An experimental platform was set up. Based on the theoretical model of infrared imaging system and MTF curve obtained by detected parameters, the feasibility of the proposed method was verified and the effect of changed edge angle on the MFT measuring accuracy was analyzed. Experimental results show that the measurement accuracy of MTF curve obtained by the method is 0.010, the measuring repeat accuracy is 0.008, and the edge angle should be kept between 2 °and 10 °. It concludes that the method effectively reduces the influences of the edge angle measurement error and noise on the MTF measurement, and the measuring results show good measurement repeatability.