The researches on the effect of liquid additives on the improvement of processing characteristics and physical properties of crosslinked polyethylene (XLPE) insulation materials for high-voltage cables are insufficient. In this paper, liquid crosslinking agents and antioxidants were taken as research objects. The effects and mechanisms of liquid additives on the scorch resistance, gel content, crosslinking by-products, thermodynamic and dielectric properties of XLPE were discussed. The results show that the liquid cross-linking agent 2,5-dimethyl-2,5-bis(tert-butyl peroxide)hexane with a high decomposition temperature and long-chain alkyl groups involved in crosslinking reaction can simultaneously enhance the scorch resistance and gel content of XLPE insulation materials, and reduce the content of crosslinking by-products. Meanwhile, the liquid antioxidants α-tocopherol and sorbitol exhibit excellent compatibility, they can form temperature-dependent proton transfer interactions, which can improve the anti-scorch performance of XLPE without compromising its gel content. Additionally, these liquid crosslinking agent and antioxidants can improve the breakdown performance of XLPE insulation while maintaining its thermodynamic properties. This study proves the feasibility of applying liquid additives to XLPE insulation for high-voltage cable.