Sea water velocity measurements are usually measured using the time-of-flight（ TOF） method

which measures the time taken by ultrasound to pass a known distance. Aiming at the requirement of high precision measurement

a method of ultrasonic flight velocity measurement based on nonlinear echo envelop model is studied. Firstly

ultrasonic receiver signal model is established

and then construct the optimal objective function of least squares method. Finally

the optimal value of the ultrasonic time-of-transmission is obtained by improved particle swarm optimization algorithm and obtain the ultrasonic transmission velocity. The experimental platform of sound velocity measurement is constructed

and the ultrasonic flight time is measured after the pure water is injected into the constant temperature flume

difference between the speed of sound and the speed of sound measured by a certain speedometer is less than 0. 15 m/s. The experimental results show that the method of ultrasonic velocity measurement based on nonlinear echo envelope model is feasible.