Abstract: The research on the division of the emergency planning zone size of small reactor was introduced. The fission product release fractions of floating nuclear power plant ACP100S and AP1000 to the environment were compared under the same accident sequence. By choosing an envelope severe accident source term, the emergency planning zone size of ACP100S was estimated. It shows that the release fraction of ACP100S is little smaller than that of AP1000 under the same accident sequence. With the smaller initial core inventory of ACP100S, the smaller source term of ACP100S releases to the environment. On the 500 m distance, two days effective dose and seven days effective dose and thyroid dose are smaller than the relative interval levels. It is appropriate to define the radius of 500 m for the ACP100S plume emergency planning zone and its off-site emergency could be eliminated.