PSV (Particle Streak Velocimetry) calculates the particle velocity by determining the particle trajectory length during exposure time, and the key to this method is the determination of the particle trajectory length. In order to improve the efficiency of determining trajectory length, a one-dimensional (1D) particle trajectory length calculation method is proposed. This method first determines the particle motion direction for each particle trajectory by using the least squares method with gray value weight. Then, the coordinates of the starting and ending points in the direction where the trajectory is longer are obtained by fitting the sum gray value of particle trajectory in the direction where the trajectory is shorter. Subsequently, by combining with the previously determined motion direction of the particle trajectory, the length of the particle trajectory can be determined. Image simulation and experimental results show that the computational efficiency of the algorithm is about 8 times that of the traditional algorithm, and the computational accuracy of the algorithm is better than 3% when the length of the particle trajectory is greater than 20 pixels under the conditions simulated.