For the life cycle of the heat exchanger network (HEN)，the heat exchanger performance steps down and energy consumption steps up because of network equipment aging and other factors.And this problem is not resolved effectively by the present margin design for the HEN.A margin optimal design method of the HEN with bypasses based on life cycle energy saving was presented.Through bypass adjustment，effective margin area was released gradually，achieving the purpose of saving energy in the life cycle.The HEN life cycle operation accumulative total cost was considered as the objective function，and at the same time the effect of fouling of heat exchangers and bypass adjustment was included to solve the best margin while satisfying the operation conditions.Lastly sustaining energy conservation was realized.The HEN of a given crude distillation unit in a refinery was treated as the specific research object，illustrating the effectiveness and application prospect of the presented method.