The naturally cooled panel-type radiators are widely used for transformer heat dissipation. However, their poor heat transfer performance also restricts the overall performance of the transformer. The chimney effect can improve the airflow rate due to the thermal pressure difference between the air inside and outside the flow channel, which is widely used to improve the flow and heat transfer of natural convection. In order to improve the heat transfer performance of panel-type radiators under natural convection, a novel radiator is developed, where wind deflectors are added at both lateral sides of the radiator, and a chimney structure is added on the top of the radiator to extend the chimney channel. Then, the fluid flow and heat transfer characteristics inside and outside the radiator are studied by numerical simulation. The results show that cooling air would be effectively sucked into the radiator from the bottom due to the pressure difference generated at the inlet and top of the chimney channel. As compared with the traditional panel radiator without the chimney structure, the airflow on the outer surface of the most cost-effective radiator is increased by 19.14 %, the cooling capacity is increased by 14.76 %, and the oil temperature inside the transformer is reduced to 6.72 °C.