Abstract: This paper follows the very approach of system engineering to the complicated process system-synthetic ammonia system composed of various devices, and employs the concept of energy cell as a threshold, decomposing a complicated synthetic ammonia system into the energy cells, investigating their correlation and bringing about the essential structure characteristics of the complicated synthetic ammonia system. Thus an energy transfer-con- version model of syntnetic ammonia system is suggested. In order to determine the main direction of study for energy saving, an exergetic analysis of this suggested model is pre- sented. This paper proves that the contribution coefficient of exergetic efficiency depends not only on exergy losses but alse on the essential structure characteristics of the system to be analysed. The authors coin the essential structure characteristic to name the type of con- nexion between cells. According to the feature that whether the conversion of one composi- tion, which characterizes one function of exergy flux on the cell, into another occurs at the connexion or not, connexion is mainly divided into two types, i.e. convertible and inconvertible, Thus the traditional point of view that the main direction of study for en- ergy saving is only determined by exergy losses should be put right.