The steam network is an important part of assistant system of main processes in iron and steel factory. The fluctuation of steam emission changing with time is related to the scheduling of the steam network. Aiming at the fluctuation of steam network system， the method of stage analysis is proposed. Application of this method， the supply and demand's mathematic analysis model of steam network system is established. Making use of the law of mass conservation and conversion， the process's dynamic analysis of steam network system is divided into three sta- ges. Comparing with other methods， the results showed that it can scientific and rational reveal the fluctuating fac- tors of the steam network system. These factors are controlled that can reduce emission of steam and save energy ef- fectively.