Abstract: The contiguous area of Shanxi，Shaanxi and Inner Mongolia of China is very rich in energy resources which are in development of high strength. At the same time the unique topography and climate conditions make it the most serious soil erosion region in China with much environmental and ecological sensitivity. How to make society and ecology systems develop together with rapid economy development and avoid the usual path of so-called ‘first development with pollution then control’ is a problem which restricts the scientific development in this region. So，this paper chooses 22 indicators of economy，society and ecology systems to set up an index system （better choose relative indexes or the per capita indexes and account by changeless price to get more accurate analysis results），and then collects data information from statistical yearbook of Yulin City，Shaanxi Rrovince in 2010（which contains the newest data until 2010，published by Yulin Bureau of Statistics in Shaanxi Province，2011），after that determines their cost weights by entropy weight method to calculate comprehensive development scores of each system. Then based on the principle of the hierarchical analysis，this paper originally builds up a new mathematical model of coordinated development above the previous research findings of urban and rural coordinated development model in China，with which accounts the coordinated development degree and then analyzes the changing trend features combining with the regional conditions. At last，the result of this research shows that each one of economy，society and ecology systems had an unbalanced development progress，while the percentage of important indicators in economy system was 50.0%，in society system was 22.2%，in ecology system was 60.0%，and that 1995 was a very important time point in the development process of this area，after 1995 the development degree （index k₁） of composite systems came into a rapid growth stage while before 1995 it increased slowly. But the regional coordination degree （index k₂） presents a “U” type curve，that is，first decreased and then increased，meeting the generalized Kuznets law of environment. It also shows that the ecology system is the key factor which restricts regional overall coordinated development. Another research result is that the coordinated development degree （index K） of composite systems also had a turning point namely 2000；before 2000 the coordinated development degree （index K） rose very slowly，but after 2000 it came into a boom. Based on the research results，the suggestion to improve the regional overall coordinated development standard of economy，society and ecology systems is increasing strength on ecological infrastructure construction and to take a green development way. All in all，this paper is a bold attempt for the coordinated development among economy society and ecology systems in a particular area in order to enrich research methods of regional science. The mathematical model about coordinated development in this research is original work which is the biggest innovation point in this paper.

Key words: the contiguous area of Shanxi, Shaanxi and Inner Mongolia, coordinated development, evaluation model, Yulin City