Groundwater resources is a major component of water resources in the northwest region of China. Scientific and comprehensive evaluation for groundwater resources is the basic work to solve the problem of water resources deficiency. This paper detailedly described the method of evaluating groundwater resources in a region. Then it described the characteristics, basic principle and content of water balance method. Through collecting information in a region northwest of China, it quantitatively evaluated the groundwater resources in this area. The purpose is to utilize groundwater scientificly and reasonablely so as to hold balance of water and soil and to promote sustainable development of economy in this area.