Abstract: Fuel rod bundle is a component of fuel assembly in pressurized water reactor (PWR), whose thermal and structural characteristics are directly related to reactor safety. In this paper, the distribution of coolant flow in the 5×5 fuel rod bundle with grid spacer was analyzed by using ANSYS WORKBENCH software. The flow and heat transfer characteristics and structural deformation characteristics of fuel rod bundle were studied by the way of multi-field coupling between coolant and fuel rod bundle. The simulation results show that the grid spacer disturbs the coolant to form the transverse secondary flow and the flow around the downstream rod bundle. The peak and average velocities of multi-field coupling secondary flow are less than that in single flow field. Secondary flows and thermal stress cause rod bundle deformation, and the uneven power and flow distribution result in the axial and radial nonuniform deformation. Compared with only rod bundle, grid spacer makes the mixing flow notable, which increases the impact on fuel rods. The deformation of rod bundle is very small in situations with and without grid spacer, and rod bundle can maintain stability of its original structure.