Abstract: The injection and metering of hydrate inhibitors are effective guarantees for ensuring the smooth transmission of oil and gas flow in subsea christmas tree, avoiding production disruptions and equipment damage. For the high-precision and wide-range measurement requirements of hydrate inhibitor injection under complex sea conditions, a ultrasonic flowmeter with “π” type pipeline is design, and the high-precision timing chip TDC-GP30 is adopted to collect and measure the ultrasonic flight time in the pipeline. The ultrasonic metering model based on the time difference method is established, and the time difference signal processing of the ultrasonic transducer is carried out based on the traceless Kalman filtering algorithm. Research shows that the ultrasonic flowmeter with a “π”-shaped pipeline structure can meet the metering requirements for the injection of hydrate inhibitors in subsea christmas tree. The ultrasonic metering design is carried out by using the time difference method, which has the high-precision measurement capability. In the case of nonlinear fluctuations, the untraced Kalman filter algorithm can provide more accurate state estimation and reduce the relative error of flow measurement.