The invention discloses a heat-resisting creep-resisting ultra-high molecular weight polyethylene constant-strength fiber. The fiber comprises ultra-high molecular weight polyethylene powder, graphene, a cross-linking agent, a curing agent, an initiator and an anti-oxidant. The preparation method comprises the following steps: swelling the ultra-high molecular weight polyethylene powder by a solvent so as to form pores; then adding a composite modifying agent wetted by the cross-linking agent, the curing agent, the initiator, the anti-oxidant, the graphene and the solvent together; penetrating the composite modifying agent evenly into the ultra-high molecular weight polyethylene powder pores, and mixing evenly; and finally, carrying out irradiation crosslinking to obtain the heat-resisting creep-resisting ultra-high molecular weight polyethylene constant-strength fiber. The heat-resisting creep-resisting ultra-high molecular weight polyethylene constant-strength fiber has the comprehensive performances of heat resistance, creep resistance, abrasion resistance, impact resistance, self lubrication, corrosion resistance, low temperature resistance, hygiene, no toxicity, difficulty in adhesion and water absorption and relatively low density, is wide in applicability, and is long in service life.