With the continuous advance in the construction of urban infrastructure, the construction of underground tunnel has become an important part. However, in cities with the complex geological conditions and dense existing tunnels, the tunnel construction often faces various challenges, especially it is more difficult to undercross the existing tunnels in the close range by the mining method. In this construction, the design technological scheme, supporting measures and construction scheme are required to consider. Taking a project under construction in Guiyang as an example, starting from the design technical scheme and construction requirements, combined with the finite element analysis and the on-site monitoring data, the problem how to achieve the safe construction in close range to undercrosss the existing small-distance tunnel under the mudstone formation condition is discussed. The aim is to provide the reference and guidance for similar projects.