Abstract: During the concrete curing condition, temperature control should be conducted to ensure that crack width does not exceed the prescribed limit. The 20.450 m floor slab crack distribution was described in a nuclear power plant reactor plant, and thermal analysis of the floor slab during concrete pouring and curing period by numerical simulation was performed. The mechanism of temperature shrinkage of the floor slab was concluded and the reinforcement measures were put forward. The crack of the reactor building floor slab is due to the concrete temperature stress and structural constraint factors such as the result of comprehensive action. The floor slab thermal contraction mechanism was further analyzed and strengthen method was suggested, which can provide reference for floor slab crack strengthening design.