The paper presents a design method for low-power SF6 current transformers, introducing a specialized SF6 current transformer structure with low-power function. Additionally, the engineering performance of the design under extremely cold conditions is thoroughly analyzed. To evaluate the internal insulation level and the temperature rise feature of SF6/N2 current transformer, an electro-thermal coupling model is set up. Simulation results confirm that the SF6/N2 current transformer exhibits superior operational performance, which demonstrates the feasibility of replacing pure SF6 with an SF6/N2 gas mixture in current transformer applications. A key improvement of low-power design is the integration of the shielding and support tube, which significantly enhances the insulation margin compared to conventional basin-type insulators. The thermal analysis verifies that the SF6/N2 current transformer maintains stable operation in extremely cold environments, with temperature distribution effectively preventing liquefaction of the gas mixture. These results present the reliability of the proposed design, making it a viable solution for power systems to operate in extremely cold conditions while maintaining high insulation performance and energy efficiency.