Abstract: An acoustic emission (AE) sensor based on over fused-taper fiber coupler is introduced to detect the acoustic emission signal produced by partial discharge in transformer. The AE sensor detects the AE signal according to the turbulence caused by the acoustic wave, which may change the optical power output of the two coupling arms. The experimental results prove that such kind of sensor has a flat response to AE in 10kHz～250kHz and its typical sensitivity is 5.6×10-6V/Pa with a noise of 1.8Pa at 155kHz.