X CT scanning technique was adopted to obtain the void information of foamed light weight soil without adding natural soil, including the void volumes, void contact relations and void sphericity as well as their distributions. After comparing the results of the void structure and unconfined compression test, the influence of foamed light weight soils void structure on its macroscopic mechanical behavior was explored. The results show that specimens with high density show a narrower volume range. The void volume shows a unimodal distribution, and the peak value is in range of 005020mm3. Similarly, the void sphericity also shows a unimodal distribution with a peak value in range of 015035, which indicates that void shape is highly irregular. Void sphericity is negatively correlated with void volume, and is independent on the void type. The comparison between X CT data and unconfined compression test results shows that for the specimens with about the same density, when the void sphericity increases by 7% or the volume of connected void decreases by 5%, the compressive strength and elastic modulus of the specimens both are enhanced more than 10%, and modulus rises more significantly than unconfined compression strength.