Optical fiber-based smart sensing is a key technology for ultrasound sensing and monitoring applications. It plays a vital role in areas from laboratorial scientific research to the field non-destructive testing. However, the sensitivity of the current optical fiber acoustic sensor is limited. Hence, it is necessary to develop highly sensitive fiber-based sensors for ultrasonic/acoustic sensing. Here, we present a photonic crystal fiber-based Bragg grating sensor, which offers significant advantages and has higher performance for ultrasonic/acoustic sensing applications. In this research, the theoretical investigations of the proposed sensor are presented. The polymer material is utilized for filling into the fiber air hole structure to enhance the sensitivity. The design of the proposed device has been optimized to provide high optical quality factor to ensure high detection sensitivity, which can be used for high sensitive ultrasonic/acoustic sensing applications.