The ranging theory of two infrared systems was introduced.The influence factors of its ranging error were analyzed and the formulas of error were derived.The simulation experiments were carried out.Experimental results show that ranging error increases with the increase of angle measurement error and the baseline error;Under certain measurement accuracy of target distance,the ranging error caused by angle measurement error is related to azimuth angle and pitch angle of the target,and that caused by the baseline error is not related to the angle.