Theory of system coupling is being focused on in recent year .The complex system not only includes mass and energy flow but also economy and information flow. How to unify and compare the different flow is a key question faced on the system coupling. Based on the fact that it lacks of quantification about the theory at present, this paper puts forward the new idea about mechanism of system coupling using emergy method in the ecotone between agriculture and pasture in North China as an example, where has serious dilemma of economical and ecological. The emergy concept recognizes that different types of energies have different abilities to do work. Emergy method provides quantitative way for complex system analysis. This paper also makes proposal that the mechanism of system coupling should be based on different land use manner and procedure of emergy transformation among different subsystem, also and emergy reserving through ecosystem service. At the same time it holds that can effect the mechanism mainly includes the primary productivity, inner constitution and the scale of animal husbandry and the value of ecosystem services. This paper also tries to develop the formula of system coupling under inner circle of ecosystem.