Density functional theory calculations have been performed to study binding modes of adsorbed CX3S (X = H and F) on Pt(111) for a large range of adsorbate coverages and the consequent work function shifts. We find that these properties are all strongly correlated to the surface coverage. Depending on the molecular coverage on Pt surface, the work function shift may be as large as 0.7 eV for Pt-CH3S and 1.5 eV for Pt-CF3S with respect to the clean surface value. Two factors contribute to the work function shift: the charge transfer between the molecule and the surface, and the molecular dipole moment. While the charge transfer contribution always tend to decrease the work function, the molecular dipole moment contribution for the CH3S and CF3S cases are oppositely directed. Thus, appropriate choices of molecular components and control of surface coverage would be effective techniques to tune the work function of the metal surfaces.