Abstract: Methyl chloride is an important chemical which has the property of flammable，explosive and toxic，it will produce carbonyl chloride in heat or sparks，and it’s a hazardous chemical that poses a great threat to the safety of life and property． Property of methyl chloride associated carbonyl chloride is easy to be neglected in environmental risk assessment，and it’s a lack of research on methyl chloride leak accident’s risk at present． Taking a petrochemical plant’s methyl chloride tank as an example，we used multipuff atmospheric diffusion model to predict the risk effect of the maximum credible accident that the chlorine methane leaks in pipeline and associated with carbonyl chloride，analyzed the extent of the accident，and proposed targeted risk prevention and emergency measures that combined with the prediction results． It's helpful in providing references for the environmental risk assessment and risk emergency management which relates to the methane chloride project．