An efficient method of resisting humidity for methylene-blue-sensitized dichromated gelatin (MBDCG) holograms is reported. The method uses a viscous liquid pure poly(styrene) solution with a low degree of polymerization as the coating material. On the basis of this coating, the diffraction efficiency of treated MBDCG holograms exposed to a high-humidity (relative humidity of 80-85%) environment was not obviously changed during observation over 3 mon.