Abstract: In hydropower station lifting machines are a key specific device, which are used to lift the heavy parts, such as turbine rotor, draw-gate. Lifting machines are very large and heavy, the main supporting structures are made of metal and by non-standard design. It is difficult to perform routine examinations using conventional nondestructive testing technology such as ultrasonic and radiographic testing. Thus, an alternative method, the acoustic emission(AE) technology was introduced to test active cracks in the structures of lifting machines. The AE signals of in-service defect were collected from a load-bearing metal beam with seeded cracks. Based on statistic parameter analysis, two quantitative indexes were proposed to assess the activity and intensity of active cracks. Furthermore, the hazard degree of the defect could be evaluated. The method could provide both theoretical guide and experimental data for the routine examination and safety evaluation of large and heavy machine.