Abstract: Villages and towns are the key nodes connecting cities and serving villages, promoting the coordinated development of villages and towns and realizing the convergence and complementarity of functions of villages and towns are the key to realize the integrated development of urban and rural areas within a county. Common index is founded based on the literature review and document research library, and through the field investigation and interview to determine personality indicators, finally a three dimensions county town spatial extension effect evaluation index system is constructed from the social, economic, ecological. Then Ya'an of Sichuan Province is taken as the research area to conducte the empirical analysis ofvillages and towns spatial extension of economic effect, social effect and ecological effect. The comparison between the evaluation results and the field survey and interview shows that it can objectively and comprehensively reflect the actual situation of rural spatial expansion in Ya'an City. The index system can not only enrich the research paradigm of rural expansion effect, but also widely guide and apply to the practice of rural sustainable development.