A compact CMUT structure with movable floating gate and low noise broad bandwidth circuit is presented in this paper. The new active CMUT structure comprises two traditional CMUT electrodes and MOS transistor beneath it. Its spice model on DC analysis and transient analysis with time-varying capacitor is investigated. It exhibits low overall parasitic capacitance and low output impedance compared with traditional one. According to Cadence simulation results under 1.5Pa ultrasound pressure, the output signal amplitude is 40.1 mV, while the noise floor is 16.59 nV/Hz1/2 at 1MHz operation frequency of CMUT receiver, unit gain frequency is 488 MHz and DC gain is 74.73 dB. These results show that the proposed CMUT structure is superior to existing one and has advantages to make an interesting potential application on CMUT array imaging.