The non-steady-state model CALPUFF is applied to simulate long-distance transportation of radionuclides. The dispersion of I-131 and Cs-137 originated from Fukushima Daiichi nuclear disaster is first studied. The airborne concentration and deposition rate of both radionuclides agree well with the measured data. Then, the potential consequences are evaluated by postulating severe accidents at Daya Bay nuclear power plant (DBNPP), which widely concern the public from the Greater Bay Area of China. Typical cases for each month are simulated, considering Hong Kong, Shenzhen, Guangzhou and Macau. The results reveal that except for June to August, the northeasterly wind dominates the dispersion of radionuclides, resulting in large contaminated area in Shenzhen and Hong Kong. Continuous attention should be paid to Cs-137 accumulation at several monitoring sites. The area reaching 50 mSv/week equivalent dose (thyroid) will not exceed 35 km from DBNPP. Measures such as sheltering and evacuation should be taken at area within 5 km.