The Informal institution in Northwest Ethnic Minority Region has dual effects on rural legal construction. In the process of rural legal construction,it can make up for the defects of formal institution to reduce the cost of legal construction,and increase benefit. It also has negative influence on social function,and can’t be conducive to the social stability,development and harmony. Civil law is to be more valued,thus avoiding and hampering the implementation of national laws and even covering the operation of national laws,so it is impossible to achieve rule of law. The coordinated development of Informal institution and socio-economic development in Northwest Ethnic Minority Region will contribute to stable and harmonious social development in Northwest Region. The Informal institution in Northwest Ethnic Minority Region has dual effects on rural legal construction. In the process of rural legal construction, it can make up for the defects of formal institution to reduce the cost of legal construction, and increase benefit. It also has negative influence on social function, and can not be conducive to the social stability, development and harmony. Civil law is to be more valued, thus avoiding and hampering the implementation of national laws and even covering the operation of national laws, so it is impossible to achieve rule of law. The coordinated development of Informal institution and socio-economic development in Northwest Ethnic Minority Region will contribute to stable and harmonious social development in Northwest Region.