In order to study the settlement change of the stratum and the ultimate support pressure of the excavation surface caused by the new shield tunnel project penetrating the existing tunnel in the upper-soft and lower-hard composite stratum, an improved wedge shaped dynamic analytical model considering the variability of the stratum and the stiffness of the existing tunnel was proposed. With the background of Qingdao Metro Line 15 Phase I project, based on the engineering investigation data, the composite stratum numerical model was established by ABAQUS to analyze the settlement curves of each analytical step and each stratum during the whole process of tunneling, and at the same time to study the dynamic change law of the deformation of the arch top and arch bottom of the existing tunnel in the process, as well as the influence of elastic modulus and Poisson's ratio on the ground settlement, so that control measures for tunnel underpassing the existing tunnel were put forward in addition to the general control measures. Finally, the horizontal displacement curves under different internal friction angles and cohesion were analysed to obtain the variation rule of maximum horizontal displacement with support stress ratio, and then the ultimate support pressure was obtained. The results of the study show that the impact of the new shield tunnel on the existing tunnel and the deformation of the ground should not be ignored, compared with the surface displacement, before and after penetration of the sandwiched soil and the existing tunnel when the bottom of the arch settlement is more significant, with a maximum settlement value of 16.5 mm. The existing tunnel has a certain deformation inhibition effect on the new tunnel arch settlement and surface settlement, the surface settlement value and the width of the sinkhole when penetrating the tunnel are significantly reduced.