In this paper it had been observed experimentally that hydrogen adsorption on nano-structure sp2-bonded amorphous carbon film, which was produced on ceramic substrate by microwave plasma chemical vapor deposition system, could improve its field emission property. Considering that surface morphology is not significantly varied with hydrogen adsorption, we suggested that hydrogen adsorption on the film surface mainly lowered the work function of emitter surface. For confirmation of the above suggestion, theoretical calculation was performed to investigate work functions of graphite (0001) surfaces with different hydrogen chemisorption sites at 1/2 H coverage by using first principles method based on DFT-GGA. An asymmetric slab supercell approach with periodic boundaries had been employed to model the graphite (0 0 0 1) surface. The calculation results were in agreement with experimental conclusion.