Based on the study of ＂four-modernizations＂ of scholars at home and abroad, this paper constructed a new indicator system of industrialization, informatization, urbanization and agricultural modernization based on the data from 1998-2015 in Hunan Province. Through the methods of factor analysis, geometric mean, harmonious degree evaluation model and other methods to calculate Hunan ＂four- modernizations＂ coordinated development value. The results showed that during the period of investigation, the index of development level of Hunan＇s ＂four- modernizations＂ increased rapidly, but the index of the level of agricultural modernization lagged behind that of the other three. In 2015, the simultaneous development of Hunan＇s ＂four-modernizations＂ was at the stage of high-quality coordination. Among the factors that influenced the simultaneous and coordinated development of the ＂four- modernizations＂, taking the perspective of agricultural modernization as the starting point, the Tobit model was constracted and it could be found that among the factors of agricultural modernization, like the ratio of agricultural capital to labor, the ratio of financial support to agriculture, the number of agricultural technicians, and the effective irrigation rate that those ＂four-modernizations＂ had a positive role in promoting simultaneous and coordinated development. Natural disasters restricted the simultaneous and coordinated development of the ＂four-modernizations＂. In view of this, agricultural modernization level could be increased by these measures that agricultural needed insisting on the priority of development, and the foundation of agricultural production capacity should be consolidated, and the rural talent echelon required to be perfected, and agriculture needed the supporting of science and technology.