Fuel assembly is a core component in the reactor, and its dynamic characteristics greatly impact structural integrity evaluation. This paper takes the plate fuel assembly as research object and proposes a simplified mechanical model that can represent the dynamic characteristics of the plate fuel assembly. Basis on this, the influence of constraint modes and vibration amplitudes on the dynamics of the plate fuel assembly is studied, and the variation of frequency with vibration amplitude of the plate fuel assembly and the gap between bottom nozzle and core support plate is obtained. The results show that the frequencies of plate fuel assembly decrease with increasing vibration amplitude when the gap is closed, and the frequencies increase with increasing vibration amplitude when the gap is open. These findings can provide guidance for dynamic analysis of the plate fuel assembly.