Based on the characteristics of microstructure of foamed mixture lightweight soil using river sludge (FMLSS), the strength properties and failure mechanism of FMLSS are studied. The results show that when water content of FMLSS is constant,with the increase of bubble incorporation internal average equivalent aperture, the percentage of porosity area of FMLSS present linear increasing trend, while all of them decrease with the increase of cement incorporation. With the increase of bubble incorporation，the size uniformity of pore in FMLSS reduces, while improve with the increase of cement incorporation. The strength of FMLSS has obvious correlations with the microscopic parameters of FMLSS such as the equivalent aperture and uniformity of pore distribution. The results provided basis for the study of the strength of FMLSS and the mechanism of the damage.