The main failure modes of the strength of cement-based foamed lightweight soil and fly ash-based foamed lightweight soil under the same water-cement ratio are studied to improve their working performance. Methods The mechanical properties of cement-based foamed lightweight soil and fly ash-based foamed lightweight soil were studied through laboratory tests. Through theoretical analysis, random sample points are established, different performance functions are obtained, and the feature analysis function of histogram is compared. The fitting and comparative analysis of cement-based foamed lightweight soil and fly ash-based foamed lightweight soil are carried out, and then the support vector machine method SVM is used to calculate the failure probability of foamed lightweight soil. The results of laboratory tests and theoretical analysis showed that the damage of cement-based foamed lightweight soil and fly ash-based foamed lightweight soil was close, and the damage was concentrated at the penetration of bubble pore structure. By using Copula binary distribution function to consider the correlation of strength of foamed lightweight soil, the failure probability of cement-based foamed lightweight soil was 5.45 × 10–7 and that of fly ash-based foamed lightweight soil was 3.68 × 10–6 under the same water-cement ratio. It is concluded that the failure modes of foamed lightweight soil strength are mainly stress concentration and bubble penetration. Under the action of external load, the bubble pore structure penetration and the decrease of cement content make the stress concentration and failure of foamed lightweight soil components occur prematurely. At the same time, the increase of cement content also reduces the failure probability of foamed lightweight soil. The cement content of each foamed lightweight soil should not be less than 250 kg/m3, and the optimum content of bubble group should be 15% of cement content.