The reducer is a very important mechanical part of large-scale rotating amusement devices such as the popular big pendulum. It constantly adjusts the speed during the operation of the device and bears different loads. Due to it is difficult to disassemble after its installation, there is no effective method for the detection or online monitoring. Acoustic emission (AE) technology is an effective tool for condition monitoring and fault diagnosis of rotating machinery. AE tests for the reducer were studied in the lab. The effects of speed change and load change on the AE signals of the reducer are respectively obtained. The filed test on reducers of a big pendulum in the amusement park was carried out. The AE characteristics of the reducer with the movement of the pendulum were analyzed. The results show that AE technology will play an important role in the health monitoring of the reducer on amusement devices.