Loess exhibits high collapsibility and water sensitivity, posing persistent challenges for infrastructure safety and sustainability in the Loess Plateau. This study proposes a dual industrial by-product stabilizer, combining fly ash (FA) and desulfurization gypsum (DG), as a low-carbon alternative to lime. Unconfined compression strength (UCS) tests demonstrated that the optimal mixture (10% FA + 10% DG) increased strength more effectively than higher single-component dosages. Multi-scale microstructural analyses (FTIR, XRD, SEM, CT) revealed the coformation of calcium silicate hydrate (C-S-H) gels and ettringite (AFt), leading to a similar to 59% porosity reduction and an isotropic pore network that enhanced load transfer. Complementary discrete element simulations (PFC3D) further validated the experimental results, showing that FA-DG addition elevated coordination numbers and the proportion of strong contact forces, thereby elucidating the particle-scale mechanisms of reinforcement. Beyond mechanical benefits, a cradle-to-gate life cycle assessment indicated that FA-DG stabilization reduced global warming potential by 32.1% and primary energy demand by 21.6% compared with lime at equivalent performance. Together, these findings provide the first integrated experimental-numerical-environmental evidence of FA-DG synergy in loess stabilization, offering a scalable pathway to valorize coal-based solid wastes while advancing sustainable geotechnical engineering.