The utility model discloses a 300 kN duplex long-stick strain porcelain insulator string for +/- 400 kV direct-current lines. The 300 kN duplex long-stick strain porcelain insulator string comprises a strain hanging point armour clamp, a connecting armour clamp, a long-stick porcelain insulator string, a grading ring and a strain wire clamp, wherein the strain hanging point armour clamp is arranged at the top of the strain string and used for connection with a tower body cross arm; the grading ring is located at the middle lower part of the strain insulator string and used for equalizing the surface voltage of the insulator string; and the strain wire clamp is located at the end portion of the strain insulator string and used for hanging a transmission conductor. The insulators of the 300 kN duplex long-stick strain porcelain insulator string are long-stick porcelain insulators and double hanging points are employed for the 300 kN duplex long-stick strain porcelain insulator string; the 300 kN duplex long-stick strain porcelain insulator string for +/- 400 kV direct-current lines provided by the embodiment of the utility model is capable of meeting the requirements of the +/- 400 kV direct-current lines.