An offshore tension anode system, comprising a tension platform, a tension device, a composite cable, and a gravity type foundation base, wherein: the composite cable is integrated with auxiliary anodes and reference electrodes, the tension device is installed on the tension platform, and an end of the composite cable is connected to the tension platform through the tension device and the other end of the composite cable is sinking to a seabed by connecting with the gravity type foundation base; and the gravity type foundation base includes an upper block and a lower block, wherein the upper block and the lower block are two separate structures, a main lifting lug is arranged on the lower block, a main central hole is arranged in a center of the upper block, the upper block is penetrated through by the main lifting lug via the central hole and is placed above the lower block, and auxiliary lifting lugs are arranged on an upper surface of the upper block; wherein the upper block includes an upper plate I, a lower plate I, an external annular side wall I, an internal annular side wall I, and a group of balancing weights I, wherein the upper plate I, the lower plate I, the external annular side wall I, and the internal annular side wall I are welded to form an annular cavity I, and the balancing weights I are arranged in the annular cavity I, and the auxiliary lifting lug is arranged on the upper plate I; and the lower block includes an upper plate II, a lower plate II, an external annular side wall II, an annular apron plate, a conical cavity plate, a filler, a group of balancing weights II, and the main lifting lug. wherein the upper plate II, the lower plate II, and the external annular side wall II are welded to form a cavity II, the balancing weights II are arranged in the cavity II, the conical cavity plate is welded to a lower surface of the lower plate II to form a conical cavity together with the lower plate II, the filler is injected into the conical cavity, and the annular apron plate is welded to an edge of the lower plate II; and the lower block is further provided with an ROV operating handle. the composite cable is integrated with auxiliary anodes and reference electrodes, the tension device is installed on the tension platform, and an end of the composite cable is connected to the tension platform through the tension device and the other end of the composite cable is sinking to a seabed by connecting with the gravity type foundation base; and the gravity type foundation base includes an upper block and a lower block, wherein the upper block and the lower block are two separate structures, a main lifting lug is arranged on the lower block, a main central hole is arranged in a center of the upper block, the upper block is penetrated through by the main lifting lug via the central hole and is placed above the lower block, and auxiliary lifting lugs are arranged on an upper surface of the upper block; wherein the upper block includes an upper plate I, a lower plate I, an external annular side wall I, an internal annular side wall I, and a group of balancing weights I, wherein the upper plate I, the lower plate I, the external annular side wall I, and the internal annular side wall I are welded to form an annular cavity I, and the balancing weights I are arranged in the annular cavity I, and the auxiliary lifting lug is arranged on the upper plate I; and the lower block includes an upper plate II, a lower plate II, an external annular side wall II, an annular apron plate, a conical cavity plate, a filler, a group of balancing weights II, and the main lifting lug. wherein the upper plate II, the lower plate II, and the external annular side wall II are welded to form a cavity II, the balancing weights II are arranged in the cavity II, the conical cavity plate is welded to a lower surface of the lower plate II to form a conical cavity together with the lower plate II, the filler is injected into the conical cavity, and the annular apron plate is welded to an edge of the lower plate II; and the lower block is further provided with an ROV operating handle.