The spray seeding substrate is an important material for the rehabilitation of mine slopes but is prone to cause problems of water and fertilizer loss when washed away by rainfall. To improve the water and fertility retention capacity of the spray seeding substrate, water-retaining materials based on mycorrhizal bran, vinyl acetate, and acrylic acid were prepared and their effects on water and fertility retention and phytoremediation of the modified spray seeding substrate were investigated by simulated rainfall washout. The results showed that the water-retaining material absorbed 123.94 g/g of water in distilled water and 84.36 g/g of water in 0.9% sodium chloride solution at an m (bacterium bran)/m (acrylic acid) ratio of 1/3, a neutralization level of 80%, a crosslinker dosage of 0.09 g, an initiator dosage of 0.36 g and a vinyl acetate dosage of 6 mL. The water-retaining material significantly improved the scour resistance and phytoremediation of the spray seeding substrate and the fibers enhanced the effectiveness of the water-retaining material. The water-retaining material reduced runoff by 18.57% in the flush-evaporation cycle and extended the initial production time; reduced nitrate, ammonium, and total phosphorus losses by 15.89%, 56.52% and 58.21% respectively; increased ryegrass germination rate, survival rate and dry weight by 76.19%, 48.45% and 33.33% respectively; increased soil total N, P, and K content by 77.48%, 33.33% and 60.66% respectively; the water content of the inter-root spray seeding substrate increased by 30.50% compared to the control. The results of the study provide a theoretical basis for solving the problem of water and fertilizer loss during the rehabilitation of mine slopes with a spray seeding substrate.