The invention relates to the technical field of nuclear fuel assemblies, and discloses an underwater test platform of a nuclear fuel assembly and a test method. The underwater test platform of the nuclear fuel assembly comprises a simulation water tank and an analysis part, wherein the analysis part comprises a transfer installation module, a detection module, a drive module and an analysis module; the installation module comprises an adjustable installation platform, a reflector substrate and a reflector base; the detection module comprises a to-be-detected object and a video camera; the drive module comprises a first drive part and a second drive part; the analysis module comprises a linear displacement sensor and a data analysis terminal. According to the underwater test platform of the nuclear fuel assembly and test method, through the installation module, the detection module, the drive module and the analysis module in the analysis part, underwater test o the nuclear fuel assembly becomes possible, and a specific design basis is provided for subsequent underwater vision measurement system of the nuclear power station fuel assembly.