The invention belongs to the technical field of wireless communication, and discloses a small wideband wide-beam back cavity planar single-arm helical antenna. The antenna is provided with a radiationsubstrate, a metal back cavity, a metal cone and a metal bottom plate. The radiation substrate is located in the topmost layer of the planar single-arm helical antenna. The metal back cavity and themetal cone are located in the middle layer of the planar single-arm helical antenna. The metal bottom plate is located in the bottommost layer of the planar single-arm helical antenna. A metal back cavity and metal cone structure and a patch resistor are adopted. The wide beam characteristic and wide band characteristic of the antenna are realized at the same time, and the technical problem of thetraditional circularly polarized antenna is solved. The influence of the antenna on the shape of the carrier and the aerodynamic indexes can be minimized while high performance of the antenna is ensured. In addition, the antenna technology of the invention can be used in space-borne and airborne synthetic aperture phased array units, and is helpful to obtain high-performance phased array antennas.