The invention relates to a support device for a circular antenna array and an assembly method of the support device, and belongs to the technical field of space physical measurement and satellite antenna structures. The device comprises three parts, namely a circular antenna array, truss support main frames and load cabins, wherein the circular antenna array comprises a circular ring and antennas; the antennas are arranged on the circular ring; the load cabins are connected with the truss support main frames through load cabin connectors; and the circular antenna array is arranged on the truss support main frames. Support for the circular antenna array is achieved; the flatness requirements, the positional accuracy requirements and the pointing accuracy requirements of a radiometer unit on the circular antenna array are ensured; the assembly method has the advantages of being simple in operation, low in cost and compact in structure; the radiation unit redundancy is reduced; detection tools for assembly are simple in structure and easy to design and process; the internal stress of the device is minimized after overall assembly; the strength, the rigidity and the stability are ensured; and the requirements on the flatness error range, the positional accuracy and the pointing accuracy in the assembly process of the circular antenna array and the support device therefor are easy to achieve.