American ginseng is a perennial herb of the genus ginseng. Its rhizome can be used as medicine, and it is a traditional and precious Chinese medicinal material. In addition, it has also been favored as a health product. Research indicates that the rhizomes, leaves, flowers and fruits of American ginseng are rich in biologically active constituents, including ginsenoside, polysaccharide, various amino acids, vitamins, volatile oil, minerals and other chemical components, which are conducive to anti-aging and enhance human immunity. Hot air drying is a key step for the processing of American ginseng, which is divided into two categories: constant condition drying and stages-varied drying. Among the two methods, the stages-varied drying process is widely used in drying production, which contains multiple drying stages and different drying conditions at each stage. The drying characteristics of the materials are fully considered in the condition setting of stages-varied drying, which is conducive to improve the drying quality and save energy in the drying process. The drying kinetic model can reveal the change rule of moisture ratio of materials and provide scientific basis for the development of drying processes. Under the condition of constant temperature drying, the accurate moisture content of materials can be obtained by directly considering the drying parameters in the existing dynamic model of hot air drying. However, since the drying conditions are different at each stage of stages-varied drying, the drying constants are different at each stage in the drying model. In traditional studies, the moisture ratio at each drying stage cannot be accurately obtained if the drying time of stages-varied drying is directly taken into account in the drying kinetics model. To solve this problem, a calculation method of drying kinetics model for the stage-varied drying was proposed, which can accurately analyze the variation of moisture ratio during the stages-varied drying process. In order to establish the method, the drying experiment of American ginseng was carried out and the experimental results were fitted. The results showed that the Modified Page model was the best drying kinetic model for the hot-air drying of American ginseng. The partial regression coefficient was obtained by linear regression analysis of drying conditions and drying constants. Based on the partial regression results, the stage-varied drying process of American ginseng was analyzed and the drying kinetics model for each stage of American ginseng stages-varied drying was obtained. By using the proposed calculation method, the change of moisture ratio in the stages-varied drying process of American ginseng was calculated and compared with the results of the American ginseng stages-varied hot-air drying experiment. It was found that the maximum relative error between the calculation results and the experimental results was 7.44%, the average relative error was only 1.78%. The results indicate that the proposed stages-varied drying kinetic model calculation method can be used to analyze the change of the moisture ratio in the drying process of agricultural products.