A Fabry Perot interferometric fiber optic hydrophone formed by Parylene coating for measuring high intensity focused ultrasound (HIFU) is presented.The Fabry Perot cavity of the fiber optic hydrophone is formed by coating a Parylene film on the cleaved end of a single mode fiber by the vacuum vapor deposition method.The sensing principle and the demodulation principle of the Fabry Perot interferometric fiber optic hydrophone has been discussed.An experiment system of HIFU acoustic field has been fabricated.The experimental results show that the proposed Fabry Perot interferometric fiber optic hydrophone can accurately sense the ultrasonic signals,the non linearity of the output voltage of the fiber optic hydrophone relative to the voltage applied to the ultrasonic transducer is less than 0.01,and the results sensed by the developed Fabry Perot interferometric fiber optic hydrophone agree with those sensed by the PVDF needle hydrophone.