The original Alishan Forest Railway main line was located on the colluviums, which, after the Typhoon Morakot, caused catastrophic erosion and landslide, resulting in some sections of serious embankment collapses. In order to make the Alishan Forest Railway back to service as soon as possible in the safest and most economical scenario, the rehabilitation principle of "conforming to nature, respecting nature and not against nature" was developed using the bypass tunnels to avoid large-scale collapse zone, connecting the existing intact tunnel to get the maximum benefits, adopting a direct approach to reduce the amount of the body, proposing optimal design on large-scale collapse of the portal area, setting up dumping field in the work area to ensure the duration and reduce the environmental impact measures.