The plasma of CH4/H2 was diagnosed with optical emission spectroscopy on a high-pressure microwave plasma apparatus at 2.45 GHz. The existing radicals in the plasma and the influence of the methane concentration on radical concentration and distribution were researched. The results indicate that the radicals of CH, Hα, Hβ, Hγ, C2 and little impurity atom Mo exist in the plasma. The intensity of emission spectrum of the radicals increases with the increase in the methane concentration, especially the intensity of C2 has a notable increase. The ratio of the intensity of the CH to Hα is unchanging with the increase in methane concentration, while that of C2 to Hα has a marked increase. The uniformity of the space distribution of the radicals becomes worse with the increase in methane concentration.