A membrane-free all fiber acoustic sensors based on a fiber-optic Fabry-Perot cavity is proposed and demonstrated. The acoustic response properties of the proposed sensor were exploited and the experimental results show that under the noise limit, the minimum acoustic pressure of the sensor can be detected up to 38.9 μPa/Hz1/2. Meanwhile, the sensing probe has a good temperature stability benefiting from its all-fiber structure. Compared with the sensors based on the acoustic sensing diaphragm, the proposed sensor possesses a simple structure, easy to make and has good linear response to acoustic pressure, which implies that such sensors can be widely used as microphone in applications of high acoustic pressure fields.