The invention discloses a high power capacity dual-branch helical antenna with a back cavity. The high power capacity dual-branch helical antenna comprises a coaxial inner conductor, a vertical extension segment, a first helical branch, a second helical branch, a coaxial outer conductor, a back cavity baseplate and a back cavity side wall, wherein the vertical extension segment is connected with the upper end of the coaxial inner conductor; the first helical branch is connected with the side wall of the vertical extension segment and uses the axis of the vertical extension segment as a central helix; the second helical branch is connected with the top of the vertical extension segment and uses the axis of the vertical extension segment as the central helix; the coaxial outer conductor is arranged on the periphery of the coaxial inner conductor; the back cavity baseplate is connected with the top of the coaxial outer conductor and is arranged on the periphery of the coaxial inner conductor; and the back cavity side wall is connected with the top of the back cavity baseplate and is arranged on the periphery of the vertical extension segment. The high power capacity dual-branch helical antenna realizes microwave radiation by using the manner of dual-branch helical series connection, and helical branches at two ends are led out at different positions on the vertical extension segment smartly, so that high-frequency current is shunted into two parts, the problem of electric field concentration in an original single-branch helical antenna is effectively solved, and the power capacity of the helical antenna can be obviously improved through optimal design.