Abstract: The fundamental development direction of agriculture in China is agricultural modernization, which is also the foundation and support of national modernization. Promoting the development of agricultural modernization is of great significance to ensure the critical progress of national food security and rural revitalization. Applying the comprehensive index method, this paper constructed a measurement index system, including 4 dimensions: agricultural production input, agricultural output level, rural socioeconomic development, and agricultural ecological environment quality, and evaluated the agricultural modernization level of Gansu Province from 1990 to 2015. In addition, this paper also analyzed the constraint factors of agricultural modernization in Gansu Province by using the constraint degree formula. Results showed that: the development level of agricultural modernization in Gansu Province had been greatly improved. In 1990, the comprehensive index of agricultural modernization was only 0.124, in the preparation period. In 2011, it entered into the initial stage and showed a steady growing trend. By 2015, the comprehensive index had increased to 0.479, which was in the transitional period from the initial to the preliminary realization stage. However, the overall level of modernization was still low. The modernization of agricultural production input was basically achieved and the agricultural output level was in the initial stage. While the rural socioeconomic development was still in the preparation stage and still a long way to go for agricultural ecological environment governance. In 2015, the constraints of agricultural modernization in Gansu Province mainly concentrated on the agricultural output level, with constraint degree of 57.53%, followed by agricultural production input, rural socioeconomic development, and agricultural ecological environment quality, and the constraint degrees were 15.99%, 15.16% and 11.31% respectively. This paper also identified four main factors constraining the sustainable development of agricultural modernization, including the output growth rate, soil erosion control, fertilizer application, and household operating income of rural residents. In summary, this paper also proposed the following policy suggestions to promote the sustainable development of agricultural modernization in Gansu Province: cultivating new agricultural business entities, building new agricultural business system to promote agricultural transformation and upgrading, enhancing diversified rural industrial integration entities to promote the transformation of agricultural production from the first industry to the convergence of three industries, and promoting the transformation from normal agricultural development to green and innovative development under the guidance of the “Two Mountains” theory.