Abstract: Based on the microwave plasma module of COMSOL Multiphysics, a numerical model of hydrogen plasma inside a MPCVD reactor was built to investigate the effect of the addition of an annular molybdenum holder at the outer side of the substrate and different Δh (the height difference between molybdenum holder and the substrate) on the distribution of plasma at the surface of the substrate. The uniformity of the plasma distribution was quantitatively analyzed by using the coefficient of variation, and the surface morphology of the diamond coating was analyzed by using SEM. The results show that when Δh = 0 mm, the uniformity of plasma distribution is optimal, and the coating morphology is significantly improved compared with that without molybdenum holder. When Δh < 0 mm, the uniformity of plasma distribution increases with the increase of Δh; when Δh >0 mm, the uniformity decreases.