Abstract: Dry-discharge iron tailings in northern semi-arid mining areas are characterized by coarse particles, poor structural stability, and insufficient nutrient supply. This study used dry-discharge iron tailings collected from Longguan Town in the Zhangxuan mining area as the test materials. We conducted a pot experiment with three factors, namely, the tailings-to-soil mass ratio, humic acid addition proportion, and inorganic fertilizer level. Tall fescue (Festuca arundinacea) was selected as the indicator plant. The structural properties, fertility indices and plant growth responses were measured, and the substrate improvement effects were evaluated using Z-score standardization and principal component analysis. The applicability of existing quality thresholds was further assessed and functionally calibrated based on plant growth responses. The results showed that increasing the soil proportion improved the particle-size distribution of the substrate, shifted the substrate texture from loamy sand toward sandy loam and silt loam, reduced bulk density, and increased the content of water-stable aggregates. Humic acid and inorganic fertilizers markedly increased the contents of total nitrogen, alkali-hydrolysable nitrogen, available phosphorus, and organic matter. Festuca arundinacea exhibited obvious responses to substrate reconstruction and nutrient supply. In untreated tailings, plant height, emergence rate, and total biomass were 10.67 cm, 44.0%, and 0.43 g, respectively, whereas the maximum values after amendment reached 19.67 cm, 93.7%, and 1.42 g, respectively. The comprehensive evaluation revealed that the treatment with a tailings-to-soil mass ratio of 5∶5, 2.5% humic acid addition and F2 inorganic fertilizer achieved the highest score of 1.81, and 18 out of 36 amended substrates performed better than the control soil (CK). Among the 18 treatments with higher scores than CK soil, 16 treatments (88.9%) failed to meet the initial bulk density threshold, and none of the treatments reached the initial Grade-Ⅱ standard for water-stable aggregates. Therefore, the current nutrient thresholds were generally instructive for evaluating nutrient indicators, whereas the indicators of bulk density and water-stable aggregates were overly strict for the early establishment of herbaceous plants on dry-discharge iron tailings in the Zhangxuan mining area. It is recommended that in engineering applications, the level-Ⅰ and level-Ⅱ functional calibration ranges of bulk density for this type of substrate be set to 1.35-1.55 g/cm³ and 1.55-1.70 g/cm³, respectively, and those for water-stable aggregates be set to ≥10% and 5%-10%, respectively. This study links formulation optimization, plant response, and threshold calibration, highlighting that the evaluation of vegetative substrates should shift from single-index compliance toward functional satisfaction for early vegetation establishment. The study provides a basis for quality control in the ecological restoration of dry-discharge iron tailings in the Zhangxuan mining area and similar semi-arid mining regions.