Micro-electromechanical systems （MEMS）-based extrinsic optic fiber Fabry-Perot ultrasonic sensors have the characteristic of anti-electromagnetic interference. They have features such as long signal transmission distance， small size， and light weight， as well as good detection and positioning ability for ultrasonic signals released by partial discharge， Owing to these properties， they have potential for numerous application prospects. Currently， most of the sensitive structures that have been reported to use this type of sensor typically have completely circular diaphragms and are relatively simple to process. However， the temperature difference or pressure imbalance between the inside and outside leads to deviations. Thus， reducing or eliminating this effect is a basic prerequisite for promoting the industrial application of sensors. In this paper， we propose an optic fiber Fabry-Perot ultrasonic sensor based on a porous sensing diaphragm. The sensor was manufactured using the MEMS process with a thickness of only 5 μm. The results showed that the sensor had a good ultrasonic response in liquids. Moreover， the static pressure sensitivity could reach 1.25 V/Pa， and the performance of the distance decay and directional response was consistent with that in air. Additionally， the pores could prevent the air pressure imbalance inside and outside the sensing diaphragm. Therefore， the proposed sensor has potential for applications in the detection of partial discharge in liquids.