In light of the severe environmental issues associated with traditional SF6 gas-insulated ring main units, particularly their high greenhouse effect and potential climate hazards, the power industry is gradually phasing out such equipment in favor of environmentally-friendly gas-insulated devices. However, existing eco-friendly gas-insulated ring main units still face the risk of gas leakage, leading to reduced insulation capability, while the demand for equipment miniaturization also presents challenges in structural design. To address these issues, this paper proposes a miniaturized zero-pressure differential composite insulation ring main unit, using dry air as the primary insulating medium. Through optimized structural design and electric field distribution, stable insulation performance is achieved under standard atmospheric pressure. Experimental validation shows that this design performs excellently in withstand voltage, partial discharge, and lightning impulse tests. It has been successfully applied in real-world projects in high-altitude and humid environments, demonstrating its advantages in both environmental protection and reliability.