Abstract: To address the stability problem of slurry trench excavation for diaphragm walls in heterogeneous composite soil layers, a stability analysis approach based on the adaptive upper-bound finite element method is adopted. An adaptive numerical model incorporating second-order cone programming and high-order finite elements is developed, and a system of dimensionless parameters is established to systematically investigate the influence of weak soil layer distribution on the critical slurry coefficient C and the evolution of critical failure surfaces. The results indicate that in an upper-soft-lower-hard stratigraphy, C exhibits a nonlinear increase with the thickness ratio k of the weak layer, where the failure mode combines localized shear and toe-type failure characteristics, while in an upper-hard-lower-soft stratigraphy, the failure surface becomes significantly constricted. The slope of the critical failure surface is positively correlated with the internal friction angle of the corresponding soil layer, and the predicted failure morphology shows strong agreement with previously published results. This study provides an efficient and reliable analytical method for evaluating trench stability in weak soil environments.