To address the problems of multiple weak failure surfaces,unclear failure mechanisms and complex instability modes existing at the interface between bucket foundations and soil in deep composite foundations,the finite element strength reduction method is adopted.The numerical model is validated by field measured data and geotechnical in-situ test results,and the effects of parameters such as the area replacement ratio of sand columns,the penetration depth of the lower bucket and the compartment arrangement of the lower bucket on foundation stability are systematically analyzed.The calculation results indicate that the failure surface at the bottom of the lower bucket is not a horizontal sliding surface as assumed in the specification,but a curved surface intruding into the bucket compartments.The depth of the lower bucket penetrating into the sand column composite foundation has a significant impact on the overall stability,when the penetration depth reaches 1 m,the safety factor is increased by about 1 time compared with the non-penetration condition,and the stability reaches the optimal level at this time.The increase in the sand column replacement ratio can improve the safety factor,but the growth rate tends to flatten out when it exceeds 35%,indicating that 35% is the optimal replacement ratio.Reducing the compartment width is conducive to improving stability.When the width decreases to 13.3 m,the growth of the safety factor slows down,and 10.0 m is the optimal compartment setting.This study reveals the curved failure mechanism of the bucket foundation and provides a theoretical basis for the optimization of key parameters.