Abstract: To address insufficient range and step speed variation of a lightweight torpedo in sea trial, the fault trees of insufficient range and step speed drop are established, and the causes of the abnorma l phenomena are analyzed relating to the power system,the homing system and the power supply system from the aspects of whole torpedo’s power supple switching, enery conservation,noise analysis and torque balance. The conclusions are drawn that the insufficient tange is owing to gas leakage of steam d istributing valve, and the step speed variation is due to power supply switching. Some corresponding counterm easures are suggested for the faults.