Research purposes： Although lots of tunnels have been built in loess regions, there is no professional specification or regulations were proposed yet for design of loess tunnel expect 11 code for building construction in collapsible loess regions GB 50025 - 200411. During the process of traditional tunnel foundation treatment in loess region, the construction is difficult , low and difficulty and high - risk. In terms of the study reports of Zhengzhou - Xian and Baoji - Lanzhou dedicated line , the collapsibility conditions and mechanical behavior of tunnel engineering is different from that of civil engineering. So a new collapsible mechanism and treatment of loss tunnel should be proposed more than quoting the old specification in civil engineering. Research conclusions ： (1) Internal factors of collapsible loess is determined by its special honeycomb structure. Meanwhile, its inducing factors and necessary conditions depend on applied loading and water. (2) Tunnel foundation induced by tunnel excavation is unloaded, contrary to that of civil engineering. (3) Calculation zero point of collapsible loess thickness start from the bottom of tunnel ( other than that in civil engineering start from ground) excluding from the contribution of soil from both sides and top of tunnel. (4) The calculation of collapse deformation is equal to an accumulative value of al soil layers beneath the tunnel foundation and classification of collapsibility is divided into three gradations based on collapse deformation. (5) Tunnel portal is generally necessary to be treated for collapsibility, corresponding technical principles about various tunnel foundation treatment are also subordinated to the 11 technical code for railway tunnel in loess Q/CR 9511 - 2014" and the achievements have been applied to the railway tunnel construction such as Baoji - Lanzhou dedicated line.