Impact of acid rain on foamed light soil with river sludge (FMLSS)was studied by percolation method. Physical and mechanical indexes of specimens, pH value and Ca2+ concentration of leachate, as well as the change of mineral composition of FMLSS after acid rain infiltration were measured during test, to investigate the influence of acid rain on properties of FMLSS. Experimental results show that acid rain results in the hydrolysis of minerals and dissolution in the form of Ca2+ ions leading to neutralization of FMLSS. It is found that the stronger the acidity of simulated acid rain, the higher the degree of neutralization. At the same time, new minerals are generated in the process of neutralization, which changes the relative content of minerals in the mixed soil and leads to the changes of physical and mechanical properties. Compared with those in the normal water condition, moisture content and density increase, while the peak strength decreases. In addition, the failure mode evolves to the brittle type after acid rain. Based on the results, it can be safely arrived at that the stronger the rain acidity, the greater the degree of physical and mechanical changes.