Abstract: To address the challenges of poor flowability, difficult processability, and deteriorated mechanical properties in highly filled polymer composites, a simple and cost-effective modification strategy is proposed by the direct incorporation of ionic liquid (IL) with excellent flowability. An ionic liquid, 1-butyl-3-methylimidazolium phosphate (BMIMPA), was synthesized by a two-step method. A series of highly filled magnesium hydroxide (MH)/linear low-density polyethylene (LLDPE) composites (the mass ratio of MH to LLDPE was 60:40) containing various amounts of BMIMPA were then prepared by melt blending. The investigation of their processing, rheological, mechanical and flame retardant properties revealed that the processing properties and impact toughness were significantly improved with the introduction of BMIMPA, while maintaining the flame retardant properties of the composites. Notably, when the addition of BMIMPA was 5%, the equilibrium torque was reduced by 9.7%, the melt flow index was increased by 119%, and impact toughness was increased by 69%. This was attributed to the improved dispersion of BMIMPA on MH in the matrix and its good lubricating effect. This study provides a new idea for the application of IL in highly filled polymer composites.