The tunnel safety factors under different construction cases are calculated by means of strength reduction method. The variation rules of tunnel safety factor vs. relationship between rock and soil interface and tunnel working face are obtained. The deformation and stress of tunnel support structure under different construction cases are analyzed by means of numerical calculation; and then the excavation method conversion time is decided. The study results show that: 1) The variation speed of tunnel safety factor becomes slow when the distance between crown roof and siltite is larger than 4 m. 2) The deformation and stress of support structure becomes stable when the distance between crown roof and siltite is larger than 3 m. 3) When the distance between crown roof and siltite reaches 4 m, it is the time for construction method conversion.