The uniformity of polarization distribution in the cross section of the beam is a key factor affecting the quality of the radially polarized beam and its practical application. The polarization purity of the radially polarized beam was measured and compared by the PBS measurement method, the slit method and the S-wave plate method, the uniformity of the polarization state of the radially polarized beam in the cross section was analyzed. In the process of measuring the radially polarized beam by the PBS measurement method and the slit method, the expression of the radial polarization purity was given, and the purity of the radially polarized beam was determined to be 93.4% and 84.1%, respectively. And the variance formula was used to evaluate uniform distribution of the radial polarization states. Among them, the PBS measurement method expressed the purity of the radially polarized beam more accurately, and the slit method can reflect the polarization distribution characteristics of the radially polarized beam more accurately by comparing the polarization degrees of different regions. The S-wave plate method can indirectly measure the purity of the radially polarized beam using the existing polarization analyzer in the market, and is more suitable for measuring the change of the polarization state of the radially polarized beam during the amplification process.