This paper is to measure the soil-water retention curves (SWRC) of undisturbed and compacted soil specimens and compare the differences in measured data between these two soils. The mercury intrusion porosimetry (MIP) tests are conducted to explore the difference in the pore size distributions (PSD) between the undisturbed and compacted specimens and investigate the evolution law of PSD during drying. By considering shrinkage during drying, the basic parameters of SWRC can be determined based on the PSD. The results show that the undisturbed specimens in the wide suction range exhibit a unimodal PSD. The saturated compacted-specimen exhibits a unimodal PSD, but becomes the bimodal structure obviously during further drying. The SWRC of undisturbed specimens has a typical shape of “S”, but the compacted specimens have a “horizontal stage” in the transition zone. The pore diameters governing the air entry value and residual value can be determined by the PSD of MIP tests, and then corresponding suctions can be calculated, which are consistent with the physical meaning.