流动，是物质（如铜、铝等）流动和流体流动二者所具有的基本特征，基于这个论点，简要地回顾了流体力学中研究流体流动的两种方法，即拉格朗日法和欧拉法；相应地提出了物质流分析的两种方法，即跟踪观察法和定点观察法。由于前者在文献中未见报道，因此对它进行了重点说明。强调了物质流的跟踪观察法既适用于稳态物质流（产品产量不变），也适用于非稳态物质流（产品产量增长或下降）。以钢铁产品生命周期的铁流图为例，说明了物质流的跟踪模型。在引入了物质流的非稳度后，提出了物质流各项指标的计算式，以及它们之间的相互关系。以瑞典铅酸电池系统为对象，计算了其中铅流的各项指标，并进行了必要的分析。

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.