Abstract: In order to evaluate objectively the safety of a oil tank farm, a method for evaluating the DOW's Index of fire explosion is put forward and its principle and evaluation procedure are discussed. It can quantitatively calculate the extent of risk and economic loss due to the accidents. Thus large vessels in a oil tank farm are evaluated reasonably, also, it can provide a scientific basis for prediction of fire of oil tank farm and daily management of a oil tank farm. Moreover, it is found that the method is available to screen hazard sources in order to make preparation for the deeply quantitative analysis of safety eval uation of oil tank farm. In this paper, the principle of risk assessment is discussed theoretically.