Abstract: The study on oil-diffusing law of product pipeline leaking is a key technical issue for the risk assessment of product pipeline. The authors study the diffusing process of the product pipeline leaking and establish the theoretical analysis models for the different phases of the oil diffusing, including the model of oil product streaming from orifice, the model of oil product jetting, the model of oil product evaporating, the model of oil sump enlarging, the model of oil product infiltrating. On top of these models, the degree of hazardous to public and environment should be precisely evaluated if the product pipeline is leaked. Therefore, the risk of the product pipeline leaking should be quantified more accuracy and accord with the fact, which is helpful to reduce the loss in a pipeline accident and study on the integrity management of product pipeline.