The invention relates to a device for releasing internal stress of a high-voltage cable, and a control method. The device comprises a shell, a heating pipe, an inner cover, a force application device and a V-shaped clamping plate, wherein the heating pipe is arranged at the lower part of the left end or at the lower part of the right end in the shell; the inner cover is arranged in the shell; the force application device is respectively connected with a left clamping device and a right clamping device; and two ends of the V-shaped clamping plate are respectively connected with the left clamping device and the right clamping device and used for clamping a cable joint. The device disclosed by the invention is compact in structure, reasonable in design, simple and convenient to operate, low in cost and convenient to adjust.