For the shortcomings of unknown mode purity and apparently ambiguous design idea in the literature (Danilov,2007), an improved rotating high-order mode generator based on the helically-distributed perforation (HDP) technique is presented to realize high purity conversion of coaxial TE1,1 mode to circular TE5,3 mode. According to the inhomogeneous string equation, the optimal configurations of the coaxial cavity are obtained by the numerical calculation so that the required mode can effectively resonate. Based on the pinhole diffraction theory and the electric field boundary conditions on the surface of the perfect electric conductor (PEC), the arrangement of HDP and the working principle of the mode suppressor are studied in detail. Numerical calculations and simulation results indicate that the mode purity of TE5,3 mode generator is as high as 97.4%, and the conversion efficiency is 96.1% near the center frequency of 30 GHz. Compared with its old version, the generator has the obvious advantages of high mode purity and compactness.