Autonomous underwater vehicle (AUV) is an automatic machine working autonomously without human intervention in underwater environment. It contains hundreds of components. Each component may fail unpredictably or even cause the AUV not to work. So it is strict on AUV’s reliability because of high expense for field trial and actual application. Reliability is a very important specification in designing a new heavy weight or large AUV. It needs to make a balance among cost, performance and maintenance. Reliability analysis method includes fault tree analysis (FTA), fault mode effect and compromise analysis (FMECA) and event tree analysis (ETA). Fault tree analysis is mainly used to find the basic causes in leading to the unexpected system failures or damaging danger events and became a common tool for fault diagnosis and reliability assessment.