Abstract: In view of the large calculation error of excavated subgrade earthwork in the stage of preliminary scheme design or feasibility study report, an improved method is proposed. Through the cubic curve regression model, the relationship between the average slope of natural cross slope of mountain and the cross-sectional area of excavated subgrade is established, so as to calculate the excavated subgrade earthwork. Through examples, it is verified that this method is more accurate than the traditional empirical method, which makes the earthwork quantities of excavated subgrade more in line with the reality, and provides reference for other projects.