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.