Abstract: Suction is a decisive factor to control the state of unsaturated soils. The filter paper method is an indirect method for measuring suction, which has the advantages of low cost, large measuring range and high precision. The soil samples are collected from a loess-mudstone landslide in Tianshui, Gansu Province. The strength of soil samples is very sensitive to change of soil water content, so the study on the properties of unsaturated soils of the sliding soil is essential. The equilibrium time of the filter paper is measured under wet and dry conditions. Then, the samples are tested by double-circle 203 and Whatman No. 42 filter papers under dry and wet conditions. The contact and Non-contact methods are used to measure the matrix suction and total suction respectively. The lower suction value of this kind of soil is determined by analyzing and comparing the moisture content of filter papers in low suction section. The Oringin software is used to fit the matrix suction curve of the samples, and the corresponding fitting parameters and equation are determined so as to provide a reference for further study and treatment of the landslide.