The design value of predicted displacement of surrounding rock calculated by traditional elastoplastic mechanics theory is large due to its ignorance on dilation effect during surrounding rock unloading. Hence, a theoretical calculation method of relaxation displacement of circular tunnel excavation by stratified summation method is established by considering the difference of damage and dilation coefficient of surrounding rock under different stress states; and a method for calculating the early warning value of surrounding rock excavation displacement is put forward, which takes the damage and capacity of surrounding rock at the wall of the tunnel as the control values. And then the displacement of surrounding rock of waterdiversion tunnel crossing Fenshui River is analyzed under the conditions of different grades of surrounding rocks, different buried depths, and different dilation effects (corresponding to different supporting states); and an empirical formula for calculating the early warning value of surrounding rock of circular tunnel is put forward. The calculated value of the method based on stratified summation of confining pressure displacement considering the dilation effect is more than three times larger than that calculated by elastoplastic theory, and the calculated results are more in line with the actual situation.