During the construction process of subway tunnel, maximum inclination rate of surface longitudinal settlement and release rate of surface settlement on the excavated face are significant for safety assessment of surface and underground building. Cumulative probability curves can not completely describe the characteristics of longitudinal settlement. Firstly, the characteristics of the surface longitudinal settlement caused by covered digging of subway tunnel in loess are researched. Secondly, a new function is sought to reflect the characteristics of surface longitudinal settlement. Thirdly, the maximum inclination rate of surface longitudinal settlement and the release rate of surface settlement on the excavated face are analyzed in the different regions of loess. Finally, Predicting method of surface longitudinal settlement is put forward based on the two characteristic values. The results indicate that during the construction process of subway tunnel by covered digging in the region of loess, the two characteristic values in the paleosol strata are both maximum, but on the contrary in the saturated soft loess strata. Through the rigorous mathematical derivations, a predicting analysis method is proposed considering maximum inclination rate of surface longitudinal settlement and release rate of surface settlement on the excavated face. Its sensitivity with characteristic values is analysed; and rationality of these methods is verified based on the monitoring data. It has a significant meaning to analyze and predict the influences that differential settlements on the surface have an effect on the ground and undergroud building.