Magnesium carbonate trihydrate fibers were synthesized by co-precipitation method using magnesium sulfate heptahydrate (MgSO₄·7H₂O) and potassium carbonate (K₂CO₃) as raw materials and sodium dodecylbenzene sulfonate (SDBS) as structural guiding agent. EVA composites were prepared by melt blending using silane-modified magnesium carbonate trihydrate fiber as a flame retardant. The effects of modified magnesium carbonate trihydrate fibers on the mechanical properties, thermal stability and flame retardancy of EVA composites were investigated by means of a universal testing machine, a limiting oxygen index meter (LOI), a thermogravimetric analyzer (TG), and a cone calorimeter (CCT). The results show that at 40 ℃, the stirring rate is 350 r/min, the molar ratio of MgSO₄·7H₂O to K₂CO₃ is 3∶1, and the addition amount of SDBS is 3% of the theoretical production fiber mass, MgCO₃·3H₂O fibers with good dispersibility can be prepared. The mechanical properties of EVA can be significantly improved by MgCO₃·3H₂O fibers modified by silane coupling agent, and the tensile strength of the composites is increased to 13.7 MPa and the elongation at break is increased to 176% when the addition amount is 40%. The heat release rate and smoke release rate are greatly reduced to 231 kW/m² and 0.029 m²/s, respectively.