In order to select suitable relieving amount, different scenarios of discharge sources shall be analyzed for flare calculation at offshore platform.Usually the actual size of safety valve is larger than the required size, which leads to an actual relieving amount far more higher than the required one.However calculation based on rated relieving amount will increase the flare facilities load and enlarge equipment and flare stack sizes.In order to solve this issue, HYSYS dynamic simulation combined with Flaresim dynamic calculation is adopted in this paper to obtain the trend of flare reliving volume and thermal radiation against time.The analysis result proves that flare length based on the required relieving capacity could satisfy flare relief safely.This paper can be used to guide engineering practice in certain extent.