At present, the shield construction of metro tunnels tends to be normal. However, there are some limitations in using traditional load system to calculate the force of shield tunnel in composite rock-soil layer. This study proposes the load system and calculation method of the existing tunnel segment of the shield up-span composite rock-soil layer by analyzing the advantages and disadvantages of the modified conventional method and the beam-spring model. Based on the section tunnel project of Guitang Station of Changsha Metro Line 4, the results of the beam-spring model and the proposed method are compared and analyzed by establishing the finite element model. The results show that after crossing the shield, the vertical displacement trend of the existing tunnel is obvious, and the passive resistance of the stratum at the top and bottom of the arch caused by the segment displacement of the shield tunnel cannot be ignored. The beam-spring model exaggerates the active earth pressure at the arch bottom of the shield tunnel, and does not consider that the lower rock stratum has weak inhibition on the tangential deformation of the shield tunnel segment, and its setting of the tangential spring is unreasonable. The proposed method is more in line with the stress condition of the segment of the shield tunnel in the composite rock and soil layer,which can provide a reference for similar projects.