The purpose of this research is to investigate the influence of acid pollution on the physical properties of loess. We prepared acid-contaminated loess specimens by immersing Lishi formation loess into hydrochloric acid solutions (concentrations of 0.5, 1, 2, 3, 4, 6, 8, 10 mol/L), and also prepared distilled water-immersed specimens as the control group. Through laboratory tests on the variations of specific gravity, void ratio and plasticity index of acid-contaminated Lishi loess as well as micro-structure analysis, we revealed the variation mechanism of physical properties of Lishi loess. Results demonstrated that: 1) the specific gravity of loess declined from 2.73 to 2.69, presenting a linear progressive reduction; 2) the increase of acid concentration led to a gradual increase in the void ratio of specimens from 0.77 to 1.33, showing a single exponential growth; 3) with the increase of acid concentration, the plasticity index of specimens augmented from 11.89 to 13.73 and then reduced to 7.34; 4) after acid action, the microstructure of Lishi loess changed remarkably. With the increase of acid concentration, the surface of soil particles was rougher; the skeleton particle looser; fine particle content smaller and pore area larger.