The floating nuclear power plant grid is an independent power system composed of power generation, in-station power supply and external power delivery. It has the characteristics of bad operating environment, complex fault forms, difficult equipment maintenance and sensitive power interruption. Combined with theoretical analysis and simulation experiments, this paper analyzes and studies the security problems and protection technologies of the floating nuclear power plant grid. The technologies of neutral-point grounding mode in cooperation with multi-mode grounding control, grounding protection of source and gird directly connected system, the series connected small-capacity transformer-based pre-magnetization technology are proposed. A network reconstruction method based on Petri net and MPGA is proposed. The research results can provide technical support for an engineering design and protection scheme for the floating nuclear power plant power system. This work is supported by National Natural Science Foundation of China (No. 51877089) and National Key Research and Development Program of China (No. 2017YFC0307800).