The invention discloses a device for measuring the axial ratio of a waveguide circular polarizer. An included angle between the polarization direction of an orthogonal mode coupler straight-through end and the bolt plane of the waveguide circular polarizer is 45 DEG, and the common port of the waveguide circular polarizer produces a circular polarized wave. The polarization pass direction of a monopole absorption load is consistent with the polarization directions of a circular moment transition waveguide and a third waveguide coaxial adapter to completely extract the required linear polarization energy. Microwave devices and measuring instruments are few, the installation is convenient, and the operation is simple.