Guantouling tunnel is the key project of Wenzhou-Fuzhou railway line. With 36 degree of planar angle，it crosses under Tong-San expressway tunnel. The space between the crown of building tunnel and the floor of existing tunnel is only 2.91 meter. Based on the fact of construction，the mechanical behaviour of rock tunnel excavated adjacent existing road tunnel is studied by 3D finite element numerical simulation method. The research demonstrates that deformation of surrounding rock and the disadvantage distribution of structure stress of the existing tunnel are remarkable. Therefore，it is necessary to take some reinforcement measures to guarantee safety of adjacent existing tunnel. According to the results of numerical simulation，the excavating method，support parameters，measures of vibration reduction and vibration isolation during construction are analyzed and discussed.