Abstract: Acoustic Emission Testing (AET) technique is a dynamic testing technology. It can provide real time information about flaw that produce acoustic emission source by inflicted load. It fits the case such as online monitoring and controlling, safety evaluating, and disaster forecasting. It can solve the problem which can’t be solved by means of conventional non-destructive testing. The example application on metal structures and equipment for the water conservancy and hydropower engineering projects proves that AET technique is a useful means for monitoring on-line and controlling or safety evaluating.?