In 2024, the extreme rainfall brought by Typhoon "Gemi" triggered a large number of shallow granitic residual soil landslides in Zixing, Hunan Province. To reveal the disaster-causing mechanism, in-situ physical model tests were conducted on typical slopes, and multi-field synchronous monitoring of volumetric water content, pore water pressure and displacement was carried out. Based on the test data, the equivalent permeability coefficients of preferential flow and matrix flow were inverted, and a double-porosity connectivity index was proposed to quantify the difference in permeability structure. A hydro-mechanical state evolution model of landslides was established. The results show that the seepage field of the slope has obvious double-porosity structure characteristics, and the equivalent permeability coefficient of preferential flow is about 24.5 times that of the saturated permeability coefficient of the matrix. The toe area shows a dual hydrological effect of water convergence saturation and activation of preferential channels for drainage. When the shallow effective saturation Se at the toe of the slope is greater than 1.35 and the pore pressure ratio Ru is greater than 0.35, the displacement enters the exponential acceleration stage from creep, and this dimensionless criterion effectively overcomes the dependence on the scale of the site. The evolution of the landslide successively experiences three stages: unsaturated seepage - loss of matrix suction, preferential flow connection - expansion of the transient saturated zone, and pore pressure positive feedback - accelerated strain softening, and finally forms a composite failure of tensile cracking - shearing - extrusion. The research results can provide in-situ benchmarks and theoretical basis for the prevention and control of granitic residual soil landslides.