Abstract: The pressure plate method, filter paper method, vapor equilibrium technique with saturated salt solution, transient water release and imbibition method and mercury intrusion porosimetry (MIP) tests are used to investigate the water retention behaviour and pore-size distribution of compacted complete-intense weathering mudstone specimens taken from Cenxi, Guangxi Province. Based on the results of the MIP tests experiencing two different suctions, a method for predicting the water retention curve is proposed. The test results show that the air entry value of complete-intense weathering mudstone is about 75 kPa, and the water retention curve in the full suction range can be measured by the three methods. There appears a plateau stage in SWCC. The SWCC calculated by TRIM matches the results of conventional tests. The saturated specimen exhibits a unimodal pore-size distribution, and the specimen experiencing suction of 38 MPa has a double-porosity microstructure. There are mainly intergranular pores with diameter of 10² nm to 10³ nm in compacted samples of complete-intense weathering mudstone. With the increase of the suction, the pores in this diameter range decrease. The SWCCs fitted by Van Genuchte and Fredlund & Xing are more close to the measured results accurately, and those fitted by other models cannot match the measured water retention curve correctly in the full suction range.