The optical emission spectra (atomic hydrogen (H'alpha', H'beta', H'gamma', atomic carbon C (2p3s->2p2: 'lambda'=165.7 nm) and radical CH (A2'Delta'->X2'pi':'lambda'=420-440 nm)) in the gas phase process of the diamond film growth from a gas mixture of CH[EQUATION] and H[EQUATION] by the technology of electron-assisted chemical vapor deposition (EACVD) have been investigated by using Monte Carlo simulation. The results show that the growth rate may be enhanced by the substrate bias due to the increase of atomic hydrogen concentration and the mean temperature of electrons. And a method of determining the mean temperature of electrons in the plasma in-situ is given. The strong dependence on substrate temperature of the quality of diamond film mainly attributes to the change of gas phase process near the substrate surface.