The invention discloses a medium-voltage cable insulating material and a preparation method thereof. The insulating material comprises 10-18 parts of ethylene-propylene methylene copolymer, 5-10 parts of carboxylic butadiene-styrene latex, 2-5 parts of acrylamido-2-methyl propanesulfonic acid, 4-8 parts of diphenylmethane diisocyanate, 3-7 parts of polytetramethylene ether glycol, 4-8 parts of argil, 2-6 parts of glass fiber, 4-10 parts of epichloro-hydrin rubber, 3-6 parts of tetra-polyethylene glycol monododecyl ether and 3-8 parts of triallyl cyanurate. The preparation method comprises the following steps: (1) adding the materials into a reaction kettle according to the weight and performing initial premixing; and (2) performing extrusion treatment on the material premixed in the step (1) by a screw extruder; and squeezing the extruded material into pieces which is the prepared medium-voltage cable insulating material. The prepared insulating material has high heat resisting strength and high mechanical properties; and the preparation technology has the characteristics of simplicity and high efficiency.