Theoretical and experimental studies have been performed to simulate and optimize graphene/semiconductor heterojunction solar cells. By controlling graphene layer number, tuning graphene work function and adding an antireflection film, a maximal theoretical conversion efficiency of 9.2% could be achieved. Following the theoretical optimization proposal, the Schottky junction solar cells with modified graphene films and silicon pillar arrays were fabricated and were found to give a conversion efficiency of up to 7.7%.