The invention provides a combined thin film pMUTs and a preparation method thereof. The composite membrane structure is composed of a circular membrane and an annular membrane surrounding the circumferential direction of the circular membrane and concentric with the circular membrane. The integral structure comprises an upper electrode, a vibration membrane piezoelectric driving layer, a lower electrode, a vibration membrane structure layer, a membrane support structure and a substrate structure from top to bottom in this order. When used in ultrasonic emission mode, circular thin film and annular thin film are excited with a certain phase difference, resulting in a coupling effect between a circular thin film-fluid medium-annular thin film, which can greatly increase the ultrasonic emission power of the cell. For an ultrasonic receiving mode, both circular and annular thin films vibrate to generate detectable electrical signals, and the wide-band ultrasonic receiving performance is realized by the resonant frequency deviation between circular and annular thin films in fluid medium. The combined thin film structure pMUTs provided by the invention have high emission power and wide bandwidth reception performance.