The effect of atomic hydrogen on C2H2-covered Si surface was studied by high resolution electron loss spectroscope and low energy electron diffraction, the findings was that atomic hydrogen broke up C—C bond in C2H2 and the Si—Si dimer to form C—H，Si—H respectively. According to experimental phenomena, several cluster models were devised for quantum chemistry calculation. The calculation results agreed to experiment well and the discussions on the reaction sites of C2H2 adsorbed on Si(100) surface and the role of atomic hydrogen and nucleation mechanisn of diamond hetereopitaxy on Si were performed.