Currently students' fitness and health status continually declines. Low and high intensity exercise will have a bad effect on students' physical fitness and health. In order to meet the needs of monitoring exercise intensity and encouraging students to do moderate exercise, a set of exercise intensity monitoring system based on wearable device is proposed and implemented. It uses three-axis acceleration sensor to capture the acceleration signal of human motion and preprocesses it, transmits it to the base station of GPRS via wireless Bluetooth and uploads it to the server automatically. An exercise intensity classification algorithm is designed to monitor exercise intensity, so teachers urge students to do moderately intense exercise. Details on hardware and software of the system are discussed, including wearable device, base station, a method of Flash storage and server software. The test results show that the system runs normally and safely. The system satisfies the needs of monitoring exercise intensity and promoting students to do moderate exercise.