The invention relates to the technical field of microwave transmission, and discloses an over-mode circular waveguide TM01 mode turning structure. The mode turning structure comprises an input circular waveguide, a coaxial waveguide and an output circular waveguide which are connected in sequence; the outer conductors of the input circular waveguide, the output circular waveguide and the coaxial waveguide are formed by conduits of the same radiuses; the input circular waveguide and the output circular waveguide are coaxial with the coaxial waveguide; conductor inserting plates are radially and uniformly distributed on the inner conductor of the coaxial waveguide; the conductor inserting plates are connected with the inner conductor and the outer conductor of the coaxial waveguide; the length of the conductor inserting plates is less than that of the inner conductor of the coaxial waveguide; the two ends of the inner conductor of the coaxial waveguide extend into the input circular waveguide and the output circular waveguide respectively; the coaxial waveguide is bent for an angle of by taking R as a turning radius, wherein R is greater than or equal to the outer diameter of the circular waveguide; and is greater than 0 degree and less than 180 degrees. The mode turning structure has the beneficial effects of compact structure, high transmission efficiency and high power capacity, and is applicable to turning transmission of high-power microwave.