Abstract: In order to improve the observation accuracy and efficiency of bed load experiments, the fine experimental observations of the movement and the state transition of bed sands in open channel flow were accomplished by means of underwater photography and particle tracking (UP/PTV). The reliability of the sampling data was improved by removing the interference caused by bed sands wagging, and setting the critical threshold of particle motion state as well as adapting multiple filter screening programs. By conducting series of bed load experiments under low transport intensity conditions, data including motion trajectory, velocity and step duration of bed particle was obtained, which was analyzed using a Lagrange process and the Probability Density Function (PDF) of the particle motion elements. Experimental results indicated that, the particle velocity shows a trend that increases rapidly at first and then decays slowly during the process of one single step, which is caused by inter particle collision and sweep events of near-bed turbulence. The PDF curve of particle velocity presents the Gamma function distribution with a thin tail, while the PDF curve of particle acceleration shows the Laplace distribution characteristics, influenced by the particle wagging effect. The particle velocity is closely associated with the friction velocity u_*, maintaining a ratio of 3.4 to 3.5. The joint distribution of the single step duration T_s and the single step length L_s of the particles presents an exponential function, and the exponent of the fitting curves ranges between 1.25 and 1.30. The exponent is associated with the transport intensity of bed load and the heterogeneity of the bed surface particles.