In recent years, the rapid development of urban rail transit in China has led to an increasingly dense subway network. The construction of new subway tunnels frequently requires crossing many buildings in the city and existing infrastructure such as tunnels. Especially in the construction of Wumi Road Station of Kunming Metro Line 5, the construction phenomenon of crossing the existing structure is particularly significant. Based on this background, this paper uses the combination of model test and numerical simulation to explore the construction deformation law of shield tunneling under the existing tunnel in sandy cobble stratum of Kunming subway. After in-depth research on the three key factors of crossing angle, tunnel clearance, and formation loss rate, we have drawn the following conclusion: whether using a 60°oblique crossing method or a 90° straight crossing method when passing through the existing tunnel, their final impact on the surface and the existing tunnel is roughly the same. Further analysis reveals a significant negative correlation between the final deformation of the surface and existing tunnels, as well as the net distance between tunnels. That is, the larger the net distance between tunnels, the smaller the deformation; there is a positive correlation with the formation loss rate; that is, the higher the formation loss rate, the greater the deformation. With the increase of distance, the range of vertical deformation and surface settlement will also increase. In addition, there is a linear relationship between the formation loss rate and the final deformation value.