The exploration and development of deep-water oil and gas resources have gradually become the main battlefield of China's oil and gas development, but deep-water drilling overflow is hidden, if not found in time or improperly handled will bring about significant safety hazards, which restricts the safe and efficient development of deep-water oil and gas, and the adoption of timely and effective underwater early overflow monitoring and early warning means to ensure the safety of deep-water drilling operations is of great practical significance. This paper combines the deep-water drilling process and ultrasonic Doppler early overflow monitoring mechanism and analyzes the acoustic attenuation under different excitation ultrasonic frequencies and the waveform conversion of ultrasonic waveforms at the interface between the sensor and the water separator, and the interface between the drilling fluid and the water separator. Based on this theoretical foundation, a two-dimensional real-size simulation model of a deep-water drilling water separator was established by using COMSOL software, and the acoustic response of the acoustic wave in the water separator was investigated when the conditions of ultrasonic excitation frequency, drilling rod eccentricity angle, and ultrasonic transducer incidence angle were changed. The results show that 500kHz is the optimal ultrasonic excitation frequency for early overflow monitoring in the watertight pipe; 30° is the optimal circumferential deployment angle of the acoustic wave monitoring sensors in the watertight pipe; and the optimal acoustic wave incident/reception angle of the acoustic wave monitoring sensors for early overflow monitoring in the watertight pipe is 30°. The above conclusions provide a theoretical basis for the accurate identification of underwater early overflow in deep-water drilling.