The method to detect the capture capability of photoelectric tracker affected by the external factors is analyzed. The mathematical equations that involve the relations between image contrast and the external factors which include target size, target velocity, target contrast in object space, light (dark) target are put forward. The detection environment consists of the adjustable contrast optical target device, high-speed camera, capture simulator and turntable. Characteristic analysis of the different external factors on capture capability gets fine mutual verification by the model analysis and image analysis. The experimental results show that the fitting equations is suitable for the accurate calculation between image contrast and the external factors in the background of which the spectral radiance is 3.2 W/(sr·m2). The study on the external factors and image contrast is meaningful for the evaluation of the capture capability on photoelectric tracker.