Abstract: Detection probability of a torpedo is one of the most important indexes of operational effectiveness for sub-marine torpedo weapon system. However, assessing the index in sea trial requires a big sample size, which means a huge expense of time and cost. Hence, a statistical test method of detection probability for a sub-launch torpedo is proposed according to the principle and mathematical model of torpedo shoot and guidance. This method makes full use of sea trial data in simulation, thus can effectively assess the detection probability of sub-launch torpedo with a small sample size.