Inorganic soil stabilizer represents a promising option to tackle the issues that arise from the land-filling of engineering waste spoil (EWS) due to its economic and environmental benefits. This paper investigates the mechanical and microscopic properties of magnesium oxysulfate soil stabilizer (MOSS) pastes. In addition, the mechanical properties of a novel type of engineering waste spoil-based specimens (EWSS), fabricated using MOSS to solidify high-content EWS, were systematically investigated. The results show that a larger mean particle size and BJH adsorption average pore diameter of LBM were found to accelerate the heat flow rate of the hydration reaction. Specifically, the peak heat flow and total hydration heat attained 30.25 mW/g and 283.9 J/g, respectively, under the condition of an MgO-to-MgSO₄ molar ratio of 9: 1 with LBM2 as the raw material. Furthermore, when the weight ratio of EWS rose from 5% to 80%, the compressive and flexural strengths of EWSS pastes at the 28-day curing age were reduced by 47.0% and 53.3% relative to the reference MOSS pastes.