To balance the trade-off between the measurement sensitivity and range of the Fiber Bragg Grating (FBG) ultrasonic detection sensor, a demodulation scheme based on Fabry Perot (FP) cavity and π-phase-shifted FBG (π-FBG) is proposed. The principle of demodulation based on FP cavity is analyzed and numerically simulated in detail. A group of demodulators consisting of two longitudinal modes of the FP cavity can double the system sensitivity, and has good linearity. A demodulator array can be constructed when expanding to multiple modes, which can greatly increase the system range, and effectively balance the trade-off between measurement sensitivity and range. Numerical simulation results show that this scheme is feasible under the practical parameters, and has the merits of high sensitivity and fast response speed.