The invention discloses a multifunctional thin film high-sensitivity CMUTs (Capacitive Micromachined Ultrasonic Transducers) gas sensor and a preparation method thereof. Multifunctional materials suchas graphene, molybdenum disulfide and MXenes with high elasticity modulus, gas sensibility and electrical conductivity at the same time are used as CMUTs sensitive elements, i.e., a single layer suspended thin film is simultaneously used as a CMUTs vibration thin film, an upper electrode and a sensitive material layer; and the integral design of multi-layer composite thin films such as the vibration thin film, the upper electrode and the sensitive material layer is realized. The mass of the thin film can be effectively reduced; the unit consistency and the resonant frequency are improved; andfurther, the great improvement of comprehensive performance such as CMUTs gas sensor detection limitation and detection sensitivity can be realized.