In view of the characteristics of the arid climate and simple land surface types in the northwest of China, Complementary Relationship Model (CRM) is discussed to evaluate its precision in calculation. By using the observational data from Yucheng experimental station which is located in Shangdong Province, the influence of vegetation diversity on CRM is analysed. The result shows that the distributions of vegetation diversity and no in-phase phenology are two important reasons that cause large-scale advection parameter, a primary parameter in CRM, to change with the seasons. At last, CRM is applied in the northwest of China. The real evapotranspiration from CRM is essentially uniform with the distribution of land surface types, and presents complementary characteristics compared with that from the 20 cm caliber evaporating dish, and similar seasonal change characteristic to precipitation, temperature, and phenology.