Photo-acoustic imaging is a promising modality for deep tissue imaging with high spatial resolution in the field of biology and medicine. High penetration depth and spatial resolution of the photo-acoustic imaging is achieved by means of the advantages of optical and ultrasound imaging, i.e. tightly focused beam confines ultrasound-generated region within micrometer scale and the ultrasound can propagate through tissues without significant energy loss. To enhance the detection sensitivity and penetration depth of the photo-acoustic imaging, highly sensitive ultrasound detector is greatly desired. In this study, we proposed a novel ultrasound detector employing optical frequency comb (OFC) cavity. Ultrasound generated by the excitation of tightly focused laser beam onto a sample was sensed with a part of an OFC cavity, being encoded into OFC. The spectrally encoded OFC was converted to radio-frequency by the frequency link nature of OFC. The ultrasound-encoded radio-frequency can therefore be directly measured with a high-speed photodetector. We constructed an OFC cavity for ultrasound sensing with a ring-cavity erbium-doped fiber laser. We provided a proof-of-principle demonstration of the detection of ultrasound that was generated by a transducer operating at 10 MHz. Our proposed approach will serve as a unique and powerful tool for detecting ultrasounds for photo-acoustic imaging in the future.