To explore the permeability evolution of conditioned muck in clayey sandy strata under complex high water pressure conditions and address the drawback that current evaluation criteria for the impermeability of shield conditioned muck are relatively single, a self-developed vertical permeability test system for coarse-grained soils is established according to engineering practices. This system realizes automatic monitoring and analysis of parameters including seepage pressure and seepage volume. Multi-stage water pressure permeability tests are carried out on conditioned muck with different clay mass fractions in clayey sandy strata. The variation patterns of permeability coefficient and effective stress conversion rate with water pressure and seepage time are calculated and analyzed. The main conclusions are drawn as follows: (1) The variation curve of permeability coefficient is divided into three stages: compression stage, fine particle loss stage, and stable stage. (2) The initial saturation state of conditioned muck in clayey sandy strata governs its initial permeability variation. Compression occurs preferentially under unsaturated conditions and reduces the permeability coefficient; by contrast, fine particle loss takes place first under saturated conditions and leads to an increase in the permeability coefficient. (3) An effective stress conversion rate of 20% is proposed as the evaluation criterion for the impermeability of conditioned muck. (4) Conditioned muck in clayey sandy strata presents better impermeability than that in sandy strata.