Abstract: In order to explore the homoepitaxial growth of single crystal diamond at low pressure and low rate by ECR-MPCVD, the real-time diagnosis of ECR microwave plasma was carried out with a high-speed optical fiber spectrometer, and the spatial distribution of plasma was studied, then the effects of pressure and methane concentration were investigated in detail on plasma emission spectrum. In the CH₄/H₂ atmosphere system, the kinds of groups in ECR microwave plasma were the same as those in the plasma operating at medium and high pressure basically. The intensity of plasma emission spectra of each group decreased along the direction of magnetic field intensity gradient decline, and the intensity of plasma emission spectra of each group was strongest in the magnetic field resonance region (875 Gs). When the substrate was placed in the magnetic field resonance region, the relative intensity of each group near the substrate showed a trend of increase at first and then decreased with the increase of the pressure, and when the pressure was near about 0.6 Pa, the H_α, H_β and H_γ groups had the strongest emission spectra, while the intensity peaks of CH and C₂ groups appeared near about 0.8 Pa. Then keeping the working pressure at 0.8 Pa, when the methane concentration increased from 0.5% to 8%, the intensity of H_α group remained unchanged almost, and the relative intensity of the H_β and H_γ groups showed a trend of decrease at first and then invariability, and the relative intensity of the CH and C₂ groups showed a trend of increase at first and then stabilization; I(H_α)/I(C₂) decreased sharply at first, then slowly decreased and then tended to stabilization; I(H_α)/I(CH) slowly decreased and tended to invariability; I(CH)/I(C₂) and I(H_γ)/I(H_β) remained unchanged almost. With the microwave power of 1200 W, the hydrogen flow rate of 50 mL/min, the methane concentration of 3%, the working pressure of 0.8 Pa, and the single crystal diamond seed temperature of 800℃, step growth formed on the polished surface of single crystal diamond when growing for 10 hours, and the growth rate was 200 nm/h.