The fire event on offshore platform often results in multiple equipment fires within a fire zone. The traditional method of calculating the flare relief load is obtained by summing the maximum relieving flow of all equipment under fire, which delivers conservative results. For a wide boiling point mixture such as crude oil, the actual relieving flow rate varies dynamically with time, and the multiple equipment under fire do not reach their maximum relieving flow rate at the same time. It is necessary to use the process system dynamic simulation method to study the actual relieving state. In this paper, the HYSYS software is uses to simulate the dynamic state of the process system. The dynamic process of the relieving parameters during fire relief was analyzed and sensitivity analysis of the initial liquid level and pressure was carried out as well. The results show that the initial liquid level is the main factor influencing the relieving flow rate, and the relief calculation should be performed under High-High liquid level. Compared with traditional method, the process system dynamic simulation method significantly reduces the flare relief load and optimizes the flare system design and vent line size. The result can be used as reference for projects of the same nature.