For more accurate test and evaluation of guidance system performance of ship-to-air missile using passive microwave /infrared compound guidance mode, main factors that affect guidance precision were required for analysis, In this paper, passive microwave / infrared compound guidance system's accuracy model was established, through detailed analysis on compound guidance failure case, specific impact of the intercepted target's microwave radiation and infrared characteristics on the composite guidance system's accuracy was studied. It is pointed out that the main factors which affect precision of the guidance system are the microwave radiation and infrared characteristics, which can offer reference for design of guidance system and target characteristics in test evaluation.