Off-axis reflective collimator has been used widely in capability testing of infrared imaging system.There are a lot of testing methods, and new methods are also appearing.Based on engineering experience, the testing environment composed of an off-axis reflective collimator is introduced at first.Then, characteristic frequency and angular diameter of the infrared target are analyzed.A test method is presented based on the definition of infrared performance indicators, and the test result is obtained through the method.The characteristics of the thermal infrared imager reflected by the test result are evaluated in the end.