In assumed accident condition, estimating the level and hazard of radioactive leakage helps in instructing public and nuclear staff to protect themselves with reasonable measures. Basing on educational demand, Tsinghua University’s Nuclear Engineering Virtual Simulation Laboratory and China Nuclear Power Operation Technology Corporation, Ltd. jointly developed a radiation-field-simulation (Rad-Sim) software. Virtual simulation technology enables the software to display radioactivity distribution visually with presumed accident in the model of Fangjiashan nuclear power plant. The software supports radioactivity simulation for various initial accident condition containing pipe-line rupture and station black-out in different wind direction. After calculation in CALPUFF model and MAPP-DOSE model, radioactivity level is displayed both inside and outside the containment. While air environment outside is monitored, users are capable to observe radioactivity distribution on plane of different height, or to compare radiation dispersal process in time scale. When radioactivity issue inside the building is discussed, a virtual human model is set in the virtual simulation circumstance. Operators are able to change the position of the human model and record according dose rate of the model. Rad-Sim software is applied in undergraduate education and public visit at present. Feedback suggests that the software is effective in better understanding of radiation protection for undergraduate and public. The vivid presentation of radiation field estimation capability strengthens confidence in nuclear safety for public visitors. Generally, Rad-Sim software brings new opportunities for nuclear education and nuclear safety research.