Abstract: The thermal-hydraulic behaviors in fuel assemblies of the reactor core are quite complicated when the reactor is under accident conditions and ocean conditions, which raises a higher challenge to the reactor safety. Therefore, it is necessary to study the transient flow and heat transfer characteristics in fuel assemblies. Based on particle image velocimetry (PIV) technique, combining with telecentric lens and a transistor-transistor logic (TTL), a long-term and high-resolution measurement for the complex flow field in fuel assemblies was achieved, the detailed flow field in fuel assemblies was obtained under the flow fluctuation, such as velocity distribution, turbulent intensity and Reynolds stress. The steady-state flow field was used as a reference, and the influence of acceleration for the spatial flow field in the rod bundle was analyzed. The results show that the accelerating flow improves the velocity gradient between flow layers in the rod bundle and weakens the lateral velocity and lateral turbulent intensity. In the contrary, the decelerating flow weakens the velocity gradient between flow layers in rod bundles and improves lateral velocity and lateral turbulent intensity. The experimental results are helpful to reveal the transient characteristics of the fuel assembly under unsteady conditions and provide a foundation for the design and optimization of the fuel assembly.