A highly flame-retardant insulated medium-voltage cable comprises, from inside to outside, a copper conductor, an inner shielding layer, a cross-linked polyethylene insulating layer, an outer shielding layer, a fire-resistant wrapping layer, a steel tape armor layer, and a sheath layer. The outer shielding layer includes an insulating shielding layer and a copper tape shielding layer arranged outside the insulating shielding layer. The cross-linked polyethylene insulating layer is formed from polyethylene, an anti-water tree masterbatch, a cross-linking agent, and a processing aid. The anti-water tree masterbatch comprises polyethylene, a copolymer, and an inorganic porous granular material, wherein the copolymer is poly(butadiene-vinyl pyridine) and/or poly(vinyl phosphonic acid).