Abstract: The macrovoid structures of loess make it easier to destroy and collapse after immersion, and it will influence the safety of project. This thesis is based on the collapsed loess of Xi'an metro line 5, and investigate the immersion characteristics by site immersion test. The soakaway which diameter are 23.0m is arranged in test site, the time of water injection are 39 days, and there are 19 days observation after stopping injection. The result of tests indicate: ① with the increasing of the immersion time, the indictions of moisture meter both inside and outside change successively, it varies quickly in early days of immersion and varies slowly gradually in later period, and it tends to be reduced after stopping immersion; the osmosis rate of loess in vertical direction is superior obviously to the the horizontal direction.② the shape of soaking influence boundary is like an inverted funnel, it extrudes slightly in buried soil; the influences scope of immersion area and saturated area increase gradually with the increase of depth, on the whole, saturated scope is less than immersion scope.③ Ground settlement develops symmetrically by the centre of Z0 settlement, self-weighe collapse tends to be lower with the increase of distance from the point to the edge of test pits, the settlement profile of combined tests line presents irregular sharp like U; the settlement ratio of shallow punctuation presents the characteristics of smallbigsmallstable.④ the process of collapsibility and the process of moisture diffusion are coincidental, there are four stages of collapse development: collapse delay stage, collapse mutation stage, collapse alleviation stage, collapse stability stage. The research achievements of this thesis can provide the technological supports and reference for engineering constructions from Xi'an metro line 5 and other relevant engineering.