Abstract: This paper deals with flow measurements based on ultrasonic time-of-flight estimation.The Unscented Kalman Filter（UKF） is proposed to deal with ultrasonic echo signals and echo signal envelope is acquired.Then the parameters of the envelope are used as the processed state vector.This paper improves the operation of UKF process based on the characteristics of flow measurement，and gives the condition of beginning and end of UKF iteration.Finally，the performance of UKF and its convergence rate are simulated，which proves its reliability，effectiveness and easiness to achieve along with advantages compared with the previous ones.