In order to predict the external overpressure strength of vertical doom-roof oil tank during explosion and provide reference for safety regulation in tank farm, the external field overpressure produced by the3000 m3oil tank internal explosion is estimatedby simulation scale method and TNO multi-energy method. And aimed at the subjectivity and the blindness of selecting the initial explosion intensity of multi-energy method, a new idea is brought up which is based on experiment to decide the initial explosion intensity. At the same time, the experimental value can be converted to the3000 m3oil tankexternal field overpressure value at corresponding positions by multi-energy method’s scaled distance equation, which is similar to the results of TNO multi-energy method. It is indicated that the prediction by TNO multi-energy method is accurate, while the simulation scale method is only suitable to estimate the overpressure field closed to the tank. When the oil storage condition is half of the 3 000 m3vertical tank, the initial explosion intensity to TNO multi-energy method is level 5.