With the development of infrastructure, large amounts of interchange projects appear inevitably. The railway tunnel underpassing expressway tunnel is prominent. Taking the design of a railway in mountain area of central Fujian province as an example, the influence zoning of singletrack railway tunnel underpassing expressway tunnel is studied. The study contents and conclusions are as follows: 1) The numerical experiment of railway tunnel underpassing expressway tunnel is carried out by means of Midas GTS under different clear distances and geological conditions. The influenced zones,i.e. strongly influenced zone, weakly influenced zone and noninfluenced zone,are abtained based on geometric adjacent degree and geological conditions and by using the settlement of expressway tunnel of 15 mm as a reference. The economical and rational construction countermeasure for every working point can be chose according to the influence zoning diagram. 2) The clear distance increases gradually with the deterioration of ground conditions. The demarcation point of strongly influenced zone and that of weakly influenced zone are 0.5D-1.8D theoretically; and the separating point of weakly influenced zone and that of noninfluenced zone are 1.3D-2.8D theoretically.