The growth of thin amorphous hydrogenated carbon films (a-C:H) on diamond (111) surface from the bombardment of CH2 radicals is studied using molecular dynamics simulations. Structural analysis shows that the local structure (e.g., the first coordination number of C atoms) of a-C:H depends critically on the content of hydrogen. The increase in kinetic energy of incident radicals leads to the decrease of hydrogen content, which subsequently changes the proportion of sp3 bonded C atoms in a-C:H.