Considered the Datong – Xi’an Passenger Dedicated Line, the field immersion test of big thickness collapse loess with heavy section in south of Shanxi region was carried out. Meanwhile, in typical areas such as Wenxi county of Shanxi Province, a large number of undisturbed soil samples were collected to carry out laboratory experiments, in order to obtain a series of loess mechanics parameters. The results show that the loess of this region is gradeⅡself-respect collapsible loess; Different depth of soil collapsibility appears many times, and the number of saturated yielding with the increase of soil depth will reduce; Water content will be different in different areas of water pool edge, at the same time, water infiltration within 10 meters is easier than it’s under 10 meters; The field measured of the loess whose lower limit of self-weight collapsibility is 10 meters, and it’s identical with the results of the computations ;The moisture expansion is horn, spread Angle is changes between 41 ° to 45 ° ; The cracks present a circular distribution, the width and the depth are both small; At last, using tension meter and pressure gauge to verify the validity of the collapsibility of loess . The study has greater usage value and realistic directive significance to the construction of Datong – Xi’an Passenger Dedicated Line.