The invention discloses an environmental stress crack-resistant polyethylene sheath material for a power wire and a preparation method thereof. The environmental stress crack-resistant polyethylene sheath material is prepared from the following substances in parts by weight: 80-100 parts of linear low-density polyethylene, 15-20 parts of polystyrene resin, 10-15 parts of polyphthalamide, 4-8 parts of dimethyl silicon oil, 2-3 parts of dibasic lead phosphate, 1-2 parts of calcium stearate, 1-2 parts of zinc stearate, 0.5-1 part of antioxidant 1010, 1-2 parts of antioxidant 168, 2-4 parts of ethylene distearate amide, 4-6 parts of chlorinated paraffin and 3-5 parts of pine tar. The sheath material has the advantages of excellent environmental stress crack resistance, high-low temperature resistance, acid-base resistance, salt mist resistance, ozone aging resistance, ultraviolet radiation resistance, abrasive resistance and tensile strength, is capable of working normally in a working environment at a temperature ranging from 60 DEG C below zero to 105 DEG C, is non-deformable and non-cracking in long-time use, is safe and reliable and is applicable to internal and external sheathes of overhead power wires in the outdoor.