Abstract: Background Bare red-soil slopes formed during engineering construction in the hilly regions of southern China are highly vulnerable to soil erosion, shallow surface failure, and poor vegetation recovery under steep terrain, fragile site conditions, and frequent rainfall. Spray-mixing revegetation is an effective technique for slope protection and ecological restoration, but traditional cement-based binders may increase substrate alkalinity, inhibit root development, and cause long-term environmental burdens. Seaweed polysaccharides are biodegradable materials with good gelling and water-retention properties, showing potential as an environmentally friendly substitute for cement. However, their optimal formulation, mechanical performance, vegetation compatibility, and field applicability in red-soil slope restoration still require systematic evaluation. Methods Typical red soil from Zhenghe County, Nanping City, Fujian Province, was used as the basic material. Seaweed polysaccharides were used to replace cement as the binder, and ecological fertilizer, corn-silk fiber, and water-retaining agent were added to prepare spray-mixing revegetation substrates. An L₁₆(4⁵) orthogonal experiment was designed with different dosages of the four components, and untreated red soil was used as the control. Direct shear tests, pot experiments, and physicochemical analyses were conducted to determine cohesion, internal friction angle, plant growth characteristics, bulk density, pH, and permeability coefficient. The entropy-weight method was used to comprehensively evaluate substrate performance and select the optimal formulation, which was further verified through a field spray-seeding experiment on a tower-base slope in Jianyang, Fujian Province. Results Compared with the control, all substrate treatments improved the mechanical properties of red soil. Cohesion and internal friction angle increased by 43.55% and 33.70% on average, respectively. Substrate No. 4 showed the highest cohesion of 20.84 kPa, 106.54% higher than the control, while substrate No. 8 had the highest internal friction angle of 25.23°, increasing by 76.80%. Single-factor analysis showed that suitable amounts of seaweed polysaccharides, fiber, and water-retaining agent enhanced cohesion, and the reinforcing effect of fiber increased with dosage. In terms of plant growth, survival rate, plant height, vegetation coverage, and aboveground biomass increased by 21.33%–146.81%, 15.46%–93.88%, 17.07%–85.37%, and -17.91%–320.67%, respectively, although belowground biomass was generally lower than that of the control. Bulk density and permeability coefficient decreased by 16.39%–27.79% and 10.94%–15.63%, respectively, while pH remained between 6.05 and 6.35, suitable for plant growth. The optimal formulation was 1% seaweed polysaccharides, 3% ecological fertilizer, 0.8% fiber, and 0.9% water-retaining agent. Under this formulation, plant survival rate reached 87.5%, vegetation coverage was 75%, aboveground biomass was 331.63 g/m², and cohesion was 20.84 kPa. Field validation showed that after 60 days, average plant height reached 18.13 cm, average root diameter was 0.69 mm, vegetation coverage exceeded 70%, cohesion increased to 28.38 kPa, and internal friction angle reached 32.28°. Conclusions Seaweed polysaccharides can effectively replace cement in spray-mixing revegetation substrates for red-soil slopes. The optimal formulation improved both substrate mechanical stability and vegetation restoration performance under indoor and field conditions, providing theoretical and technical support for green ecological restoration of bare red-soil slopes.