Abstract: During reactor operation, the hydraulic lift generated by coolant flow may cause fuel assemblies to bounce upward. To address this, a hold-down system must be installed at the upper nozzle of the fuel assembly, with its core components being the hold-down plate spring and corresponding fastening screws. This paper establishes a mechanical analysis model that comprehensively considers irradiation effects, temperature effects, and the mechanical properties of the spring to evaluate the mechanical behavior of the hold-down plate spring under in-reactor irradiation conditions, and to verify whether the design of the fuel assembly hold-down system meets functional requirements. Based on the above model, a mechanical analysis software for fuel assembly hold-down plate springs (referred to as “CACTUS") was developed and validated using measured data from in-pool inspection at nuclear power plants. The software calculation results can envelop the actual measured data from power plants, satisfying the conservatism requirements in engineering applications. The independent development of this software addresses the lack of domestic tools for mechanical analysis of fuel assembly hold-down plate springs, providing support for the successful loading of independently developed fuel assemblies into reactors.