Abstract: The flow rate of the primary coolant in a nuclear reactor will fluctuate during accident conditions or in floating reactors that are influenced by inertial forces in the ocean. These fluctuations may have a substantial impact on the heat transfer by the primary coolant of the reactor. In this study, combined with phase-locked PIV technology and matching index refractive (MIR) technology, the instantaneous velocity in the rod bundle channel with and without spacer grid was measured under the pulsating flow. In addition, the phase averaged velocity and RMS component distribution of different phases were analyzed. The experimental results demonstrate that for the bare rod bundle the acceleration increases the fluid velocity near the channel wall. And RMS decreases as the fluid accelerates and increases as the flow rate decelerates. For pulsation flow driven by axial pressure gradient, the u′ lags behind the v′, and both of them lag behind the change of flow rate. The strong mixing effect on the spacer grid with mixing vane greatly reduces the influence of fluid acceleration on the velocity distribution and turbulence intensity in the rod bundle channel. The experimental results are helpful to understand the mechanism of pulsating flow in the rod bundle channel more clearly.