Acoustic emission sensors are widely used in real-time health detection of equipment. The traditional piezoelectric ceramic transducers have the problems of large volume, high energy consumption, difficult integration and high cost. In this paper, a hexagonal piezoelectric film acoustic emission sensor based on ScAlN is designed. The volume is Ø18 mm×6 mm. The sensitivity of surface wave and longitudinal wave is higher than 60 dB［Ref.V/(m/s)］in the bandwidth of 40-500 kHz. After the design and packaging of the matching layer, the gear fault detection system is built, which can effectively distinguish the normal gear and the broken tooth signal. The test results show that the piezoelectric film acoustic emission sensor based on Micro-Electro-Mechanical Systems (MEMS) has both high sensitivity and miniaturization, and has high reliability and accuracy in gear fault diagnosis.