Internal flooding occurred in nuclear island threatens safe operation of the power plant by causing common-mode faults in many systems and equipment. Therefore, it is necessary to identify the potential flooding source, calculate the flooding height in the room, determine whether the safety-related Structure, System and Component (SSC) in nuclear island is affected, and the effective protective measures can be taken in the design to avoid or eliminate the potential hazard from the internal flooding in nuclear island. The internal flooding analysis simulation software for nuclear power plants (abbreviated as CNIFA) developed on the the basis of small reactor ACP100, and it is based on Lagrange system moving particle method. Users only need to import the geometric model of the building in a specified format into the software and input the relevant parameters to perform simulation. After the calculation is completed, the whole process of flooding can be observed. In order to verify the accuracy of CNIFA, the comparative calculation is carried out with the general fluid simulation software DualSPHysics, which is also based on the Lagrange particle method. By comparing the calculation results of CNIFA and DualSPHysics, the following conclusions can be drawn: the flooding path of two software are 100% consistent; the flooding range of two software are close, with a deviation of less than 10%; the flooding height of two software are close, with a maximum deviation of 0.017 cm. So that, the accuracy of CNIFA is verified.