Fire protection is a key safety issue in nuclear power plants (NPPs). The new reactor design puts forward higher requirements for safety and economy. Fire PRA (Probabilistic Risk Assessment) is beneficial in identifying weaknesses and making improvements in fire protection. However, because fire PRA relies on the integrity of design inputs, it is usually completed at a later stage of the design. Design improvements in the later stages of the final design have problems such as high construction difficulty and poor economic efficiency. This paper studies the simplified model method of fire PRA in the preliminary reactor design stage without detailed data. So that the risk informed method can be applied in the early design stage. This will achieve the forward design purpose of selecting the optimal design solution in the early design stage when it is easier to implement design improvements. This paper studies a simplified method for fire PRA technical factors. This method realizes fire PRA modeling in the preliminary design stage with incomplete equipment and cable layout information.