Abstract: Aiming at different sensing characteristics on waveform and amplitude of a π-phase-shifted fiber Bragg grating under axially and vertically incident ultrasonic wave in water, a fiber Bragg grating ultrasonic sensing model is established based on transfer matrix method. Inhomogeneous strain fields along grating under axially and vertically incident ultrasonic wave are calculated, which helps to analyze sensing characteristics of π-phase-shifted fiber Bragg grating under above two situations by using further wavelength shift results. Experiments are then performed to verify analytical results. Results show that the sensing characteristics of π-phase-shifted fiber Bragg grating are obviously affected by different inhomogeneous strain field distributions under two incident angles. Both sensing waveform and amplitude under vertically incident ultrasonic wave perform better than axially, and π phase shift plays a significant effect on sensing characteristics. With the increase of the distance between incident position and phase shift point, the response under vertically incident ultrasonic significantly declines. The research provides guidance for analyzing ultrasonic sensing mechanism of π-phase-shifted fiber Bragg grating.