Abstract: Suffusion of stratum ① in deep alluvium foundation of Luding Hydropower Station induces water gushing incident. Continuous migration of fine particles may induce skeleton deformation, and then it may threat the safety of Luding Dam, downstream cascade hydropower stations and Luding County. Particle size distribution of soil, stress and loss of fine particles may affect the skeleton deformation, but the detailed influence mechanism is not clear. In order to evaluate the influences of stratum ① suffusion on the skeleton deformation, a new high stress and large diameter soil suffusion apparatus which can simulate the stress and the characteristics of stratum ① particle size distribution is designed, and a criterion distinguishing the obvious skeleton deformation is proposed. When the volumetric strain during suffusion is greater than or equal to 1%, it indicates that the obvious skeleton deformation occurs. A new evaluation index called migration ratio of fine particles is proposed to weigh the loss of fine particles. A list of hydro-mechanical coupling suffusion tests under the extreme adverse hydraulic conditions are performed on stratum ① soil to investigate the influences of particle size distributions, stresses and losses of fine particles on the skeleton deformation. The results indicate that the cumulative volumetric strains during suffusion of all specimens range from 0.1% to 0.49%, and are lower than 1%, indicating that significant losses of fine particles in stratum ① soil do not induce obvious skeleton deformation, and suffusion of stratum ① soil cannot induce the cracks and breaking off of concrete cutoff wall and sudden dam break. The particle size distributions significantly affect the migration ratio of fine particles, hydraulic gradients initiating suffusion and failure. The less the percentage of finer than 5 mm, the larger the migration ratio of fine particles, and the lower the hydraulic gradients initiating suffusion and failure. The overburden pressure has a slight influence on the migration ratio of fine particles, but it significantly affects the hydraulic gradients initiating suffusion and failure. The larger the pressure, the larger the hydraulic gradients. The results may provide an important basis for the evaluation of deep seepage stability of Luding Dam, and an important reference for other similar projects.