In order to obtain sound velocity in liquid, an experimental method of measuring sound velocity is designed. A time-difference type ultrasonic flowmeter and normal flow checkout equipment are used to measure the flow speed of liquid in closed hydraulic pipeline simultaneously, then the measured value and the true value of the flow rate are measured, and the meter-factor of the ultrasonic flowmeter is determined. Furthermore, the relationship between the sound velocity and the meter-factor is derived under a certain condition. Based on the method, the velocity of sound in dinitrogen tetroxide (N2O4) at 7.6-19.4 ℃ and unsymmetric dimethyl hydrazine ((CH3)2NNH2) at 6.5-25.2 ℃ are measured at 0.17 MPa. It provides reference for measuring the sound velocity in other liquid.