The spectrum shift of FBG to ultrasonic wave is caused by the refractive index profile changing along the FBG, which can be attributed to nonuniform perturbation caused by strain-optic and geometric effects of ultrasonic wave. Response of FBG to the above two effects was analyzed by V-I transmission matrix model, showing high computing efficiency. Spectra response of FBG under changing ultrasonic frequencies was simulated and discussed. In experiment, the system was sensitive enough to detect ultrasonic wave from 15 kHz to 1380 kHz. These results would provide a guideline for FBG-based acoustic detection system design in a specific ultrasonic frequency.