For the negative-ion-based neutral beam injection system, the direct current (DC) high-voltage transmission line (HVTL) is the link between the radio frequency (RF) negative-ion source system and the power supply system, which not only realizes the function of the power transmission between the power supply system and the RF negative-ion source system, but also provides transmission channels for high-pressure cooling water, working gas, and the measurement and control signals needed for the operation of the RF negative-ion sources. In this study, the experimental sample for the DC HVTL is developed based on the insulation simulation design, and an insulation performance evaluation test bed of the experimental sample is designed and built. The insulation performances of the experimental sample at different SF6 gas pressures are investigated, and the leakage current laws of the experimental sample at different applied voltages and different SF6 gas pressures are obtained. The test results show that the maximum leakage current is 472 μA at a loading voltage of 500 kV, which proves that the experimental sample for the DC HVTL satisfies the requirements of the insulation design.