Based on FVCOM numerical model and with the support of relevant measurement materials at Ningbo-Zhoushan waters, the Epoxy Chloropropane has been selected as typical dissolved hazardous chemical substance to simulate the diffusion after the emergent leakage of hazardous chemical substance in the waters. According to different grades of the accidents, as is indicated by the simulation result, the concentration of epoxy chloropropane will be diffused to below safe concentration (0.02 mg/L) after 24 hours of leakage for ordinary and relatively big accidents. As to major accidents, however, there are still large areas of waters (30~190 km²) which have been polluted with concentration exceeding the safe limit after 24 hours of leakage. From the aspect of different tides time at diffusion, the diffusion is faster at ebb tide than at flood tide, and the diffusion process to below safe concentration at ebb tide is 2 to 6 hours faster than at flood tide. From the aspect of different regions of leakage, the diffusion at Cezi channel is faster than at Zhenhai port. The pollutants are basically diffused along the water channels when the leakage occurs at Cezi channel, and they will be detained along the bank of Beilun district and at Beilun and Daxie Island when the leakage occurs at Zhenahi port.