A generalized soil structure index (GSSI) was developed to predict the structure properties based on solid, liquid, and gas phases for dry land soil with the diminishing marginal productivity in the Cobb-Douglas production function for reference. GSSI is defined as GSSI=\[（XS-A）XL XG\]k, where A and K are the constants for dry land soil, the subscripts S, L and G denote solid, liquid and gas, respectively. Soil three-phase structure distance (STPSD) between the ideal 3-phase condition and that of a given soil was calculated in 2-D space. The GSSI was validated a typical black soil of Northeast of China producing potatoes both before and after subsoiling operation. Results indicate that the GSSI can effectively reflect subsoiling impact on this dynamic property. Based on this, 281 soil samples were collected and used in the data-fitting of the relationship between GSSI and STPSD, giving a highly significant linear relationship, R2 = 0.86**. It is suggested that GSSI and STPSD can be used as a quantitative index to describe soil structural change affecting soil quality.