Molecular dynamics simulations are carried out to investigate the effect of low energy CH radical on the growth of hydrogenated carbon film. The results show that the adsorption rate of CH on clear diamond(111) is about 98%, while on hydrogenated diamond (111) the adsorption rate is lower than 1%. It indicates that the selective adsorption of low energy CH radical at the unsaturated surface C site is the dominated mechanism of the hydrogenated carbon film growth in PECVD.