The common defects of hydraulic（ liquid spring） circuit breaker are oil leakage and frequent start of oil pump. The internal leakage is difficult to detect with inspection because the leakage point is hidden inside valve body. At present

such defects are considered as serious and critical defects. It needs to arrange power outages for defects elimination immediately.Based on different ways of judgement

leak detection methods are divided into direct and indirect leak detection methods. Direct leak detection method relies on people’ s work experience and subjectivity. It would be easy to cause false judgement. Indirect leak detection method includes sub-pressure test

mass/flow balance method

negative pressure wave method and ultrasonic leak detection method etc. In this paper

the feasibility and efficiency of the ultrasonic leak detection method are verified by comparing various methods of leak detection with the most common application and the actual case of oil leakage and the large number of test data.