The leakage of liquid ammonia during road transportation is chosen as the research object in this paper,and then the model of toxic gas diffusion after leakage accident is established; MATLAB is used to achieve the accident simulation and risk analysis of continuous leakage from moving and fixed point source.On the basis of different harm to human body of different concentrations of ammonia and the spread concentration curve of the ammonia,the dangerous area is divided into five zones: mild zone,moderate zone,severe zone,dead zone and instantaneous dead zone; according to the comparison between leakage diffusion processes from moving and fixed point source,following conclusions can be reached: during the leakage,area of dangerous zones increased and after the leakage,dangerous zone areas first increased and then decreased; the peak ammonia concentration from fixed source is higher than that from moving point source,whereas influence area of the former is smaller than the latter.This method can quickly simulate the scope and severity of leakage accident in road transportation and provide references for the emergency rescue after accidents.