A fiber-optic ultrasonic sensor based on cascaded Fiber Bragg Grating Fabry-Perot (FBG-FP) is proposed and experimentally demonstrated. An air bubble with a small size is formed on the end of FBG when the corrosion of FBG and Single Mode Fiber (SMF) are spliced with fiber fusion splicer. Because of the synergic effect of the cascaded FBG-FP, the sensor performs 67 dB Signal-to-Noise Ratio (SNR) to high frequency ultrasound. In addition, two models are designed to verify the chromatography ability of the sensor. The results of the present study indicate that the sensor could distinguish the interface of the water and the models within a range.