Based on the relationship between ultrasonic transmission speed and temperature in solid medium, the design of ultrasonic thermometer was carried out. High-speed high-resolution hardware circuit and software subdivision interpolation algorithm are used to exactly ascertain the end time of ultrasonic echo wave and accurately calculate propagation time of ultrasonic wave. Mathematical model between propagation time and temperature in solid medium has been established to obtain the accurate temperature of the medium by evaluating the propagation time, which can be used to measure the temperature of medium. The designed thermometer has the advantages of simple structure, realtime continuous measurement of liquid temperature, and can be widely used in temperature measurement of various high-temperature environments.