Abstract: When leakage or fire accidents occur to liquefied hydrocarbon tanks, the pressure-relieving valve could not relieve the pressure automatically, but only limit the tank body's working pressure between its onset pressure and maximum allowable working pressure. Depressuring systems are used to solve this problem. Through calculation analysis, the general requirements of API 521-2014 is not applicable to the design of depressuring systems in larger-capacity liquefied hydrocarbon tanks, a new calculation method was proposed to take the blowdown area required for the maximum flow rate at the set pressure of blowdown valve (BDV) in the fire case as the input area of dynamic depressuring module. The blowdown data of liquefied hydrocarbon tanks in the fire case within 10 h was analyzed with a 2 000 m3 liquefied hydrocarbon tank as an example. It is indicated that the maximum pressure, maximum temperature and maximum flow rate of depressuring systems all appear in the stage of fire start and decrease gradually as the depressuring process proceeds. Meanwhile, the proportion of light components in blowdown vapor declines gradually while that of heavy components rises gradually. It is concluded that the liquefied hydrocarbon tanks can be protected effectively by the depressuring systems which are designed based on this method.