Power transformers serve as critical connectors in the power system network. With the escalating transformer loads driven by global climate warming and economic expansion, the issue of heat dissipation for outdoor power transformers is intensifying. Given the limitations of existing methods for enhancing heat dissipation in outdoor power transformers, this paper introduces a novel approach based on radiative cooling. Taking a 1000 kVA-35 kV/10.5 kV three-phase oil-natural-air-natural (ONAN) transformer as the research object, the heat dissipation effect of radiative cooling technology is quantified by building an electromagnetic-thermal-flow-radiation multi-physical field coupling simulation model. The results show that under the conditions of ambient temperature of 40 ℃, solar radiation intensity of 1000 W/m2 and transformer overload of 20%, the radiative cooling technology reduces the hot spot temperature of the transformer by 5.1 K. The research results can provide guidance for the application of radiative cooling technology in the field of transformers.