Abstract: The theory and method for estimating the storage quality of apple using mechanical property indexes were studied. Gray relevancy analysis of the relationship between physicochemical properties and mechanical properties of apple was conducted after a continuous testing for those properties under refrigeration, and it was found that physicochemical property indexes had diverse effects on mechanical property ones. The mathematical model for multifactor dynamic changes of apple quality, which can determine the effects of various physicochemical property indexes on mechanical property ones, was established by using GM(1,5). Furthermore, the relationship between mechanical properties and physicochemical properties of apple was ascertained by GM(1,2). It was considered that the parameters, such as hardness, titratable acid, moisture content, and soluble solid, could be described with the maximum compression force, and the first three parameters could also be described with yielding force. In addition, the hardness could be described with Young's modulus E_0.5. In conclusion, GM(1,1) was established, which could be used to estimate the dynamic change regularity of the above two properties of apple during its storage. The usability of the model was proved by experiments. The results indicated that it was feasible to determine and estimate the storage quality of apple by using mechanical property indexes, which was very significant and useful in both theory and practice.