JRODOS (Java Real-time On-line Decision Support) is currently being developed by the German KIT company and promoted globally. Because of its good user visual interface and efficient calculation and analysis functions, it has been widely used in nuclear power plants accident consequences assessment. In this paper, the MVK data set is used to verify the simulation accuracy of the RIMPUFF atmospheric dispersion model in the JRODOS system. The verification conclusion shows that the calculation results of RIMPUFF are significantly better than other calculation models. This paper presents a case study of CAP1400 severe accidents consequence assessment in one nuclear power plant base on the JRODOS platform, the IC severe accident (SA) was chosen to give an example for the rapid assessment of nuclear accidents. The research in this paper shows that for the CAP1400 nuclear SA, the I-132, I-135, I-133 and I-131 are the main radionuclide. Under the selected typhoon weather conditions in this paper, the maximum dose rate within 48 hours is only 1.79E−03mSv/h, the maximum individual dose is only 3.06mSv, the impact area is generally within 6km downwind. Under the selected typhoon weather conditions, the plume can basically reach the monitoring point in the downwind direction within 1h, while for other monitoring points, it will take 17h to reach the points, this is mainly affected by the wind direction. The research results of this paper can be used for rapid assessment of nuclear accidents in nuclear power plants and formation of evaluation manuals, which can better guide emergency or conventional emergency drills.