In order to ascertain the relationships between young musk melon seedling and its environment, a mechanism model driven by two factors, effective temperature and water, was worked out. The model reveals the mechanism of plant height, stem diameter and main root length, the length of cotyledon, and leaf No. 1, 2, and 3, of young musk melon seedling, applying linear regression and logistic functional formulae, employing stage planting and irrigation maximum methods and taking advantage of the theory of accumulated temperature. Simultaneously, model parameters and the process of growing shape change were also analyzed and studied. The results show that plant height and stem diameter are linearly correlated with effective accumulated temperatures, and that the temperature effect model of the main root length and the length, width and area of cotyledon, leaf No. 1, 2, and 3 is logistic, while the water effective parameter is an index function. It is validated that the model is able to simulate real plant shape change truly and objectively.