The fundamental argument set forth in the paper is that flow is the common feature of fluid flow and substance flow. In view of this, a short review of two ways of describing the motion of fluid in fluid-dynamics was given: one of them was developed by Lagrange, and the other-by Euler. Correspondingly, two methods for substance flow analysis were advanced : one of them is “ following-observing ” method, and the other-“standing-observing” method. The exposition was focused on the former, because it is the new one, on which no information could be found in literature. It is emphasized that the following-observing method is applicable not only for studying steady substance flow (constant output of product), but also for unsteady substance flow (increasing or decreasing output of product). The following-observing model of substance flow was explained by using iron flow diagram in the life cycle of steel products. Introducing the degree of unsteady-state, a set of basic equations for calculating the indexes of substance flow and the relationship between them were derived. The lead flow in Swedish lead-acid battery system was analyzed as a case study, including the calculation of indexes of lead flow.