Abstract: Coal gangue, a major solid waste generated from coal mining and processing, has been increasingly explored as a component of cultivation substrates. Most existing evaluations focus on a single planting cycle, leaving it unclear whether substrates with high gangue proportions can maintain adequate water retention, mechanical stability, and nutrient supply across repeated cropping. To address this gap, this study evaluated six coal gangue–vermiculite–pumice blends (T1–T6) across four consecutive planting cycles. Coal gangue was sieved to ≤2 mm, while vermiculite and pumice were sieved to 3–6 mm; the gangue volume fraction ranged from 0% to 100%, with the remaining two components adjusted in equal proportions. Sorghum was cultivated in the first cycle, followed by ryegrass in cycles 2–4. Each treatment included four biological replicates inoculated with the arbuscular mycorrhizal fungus Rhizophagus irregularis and one uninoculated control. Plant dry weight, saturated water-holding capacity, and shear strength were determined for all treatments, whereas pH, electrical conductivity, alkali-hydrolyzable nitrogen, available phosphorus, and available potassium were additionally measured in the pure coal gangue substrate. Plant biomass decreased continuously with increasing coal gangue content. Sorghum total dry weight dropped from 15.33 g in T1 to 4.41 g in T6, and cumulative ryegrass total dry weight over cycles 2–4 reached 18.37–20.61 g in T1 but only 2.92–3.46 g in T6. Inoculated treatments generally produced higher biomass than the uninoculated control at equivalent gangue contents, although the overall declining trend persisted. Along the T1–T6 gradient, saturated water-holding capacity decreased from 150.00% to 29.14%, whereas shear strength under 100 kPa normal stress increased from 54.78 to 131.75 kPa after the first cycle. In the pure coal gangue substrate, pH dropped from 9.23 before planting to 7.04 after the fourth cycle, and shear strength decreased from 131.75 to 100.17 kPa between cycles 1 and 4. Alkali-hydrolyzable nitrogen and available phosphorus declined by 84.95% and 73.13%, respectively, whereas available potassium increased from 137.00 to 348.63 mg/kg. The suitability of coal gangue–based substrates for crop cultivation depends on a coordinated balance among water retention, mechanical properties, and nutrient supply, rather than on the gangue proportion alone. Formulation design should account for performance dynamics during successive cropping rather than relying on single-cycle outcomes; higher gangue utilization is justified only when the substrate maintains sufficient plant-supporting capacity. Three limitations should be noted: (i) because the proportions of all three components varied simultaneously, differences among formulations cannot be attributed solely to independent coal gangue effects; (ii) the uninoculated control lacked biological replication, rendering inoculation effects descriptive; and (iii) the absence of an unplanted control prevents isolating plant-driven effects from temporal substrate weathering. This study provides a theoretical basis and data support for the long-term, safe utilization of coal gangue in soilless cultivation and the precise regulation of ecological substrates.