Monitoring is an important means to obtain the dynamic behavior of the surrounding rock mass during tunnel construction. The ultimate displacement of the surrounding rock mass can be predicted by regression analysis on the monitoring data, which can provide effective guidance for the design and construction of tunnels. The types of functions that can be used as regression functions are discussed in the paper. It is theoretically reasonable and practically effective to take function U=A-Bln(1/t+C) as a regression function. The principle for regression analysis function selection is proposed in the paper. The fitted residual sum of the squares can be minimized by varying constant C so that the regression results can be more realistic. The paper can provide reference for the analysis of the monitoring results of tunnel works.