Water retention procedures by sealing are often suggested during the laboratory preparation of calcium oxalate protective films. However, water retention by sealing is impractical in application and is not always necessary under certain circumstances. In this paper, calcium oxalate was prepared without water retention in a humid environment and the protective performance was investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), acid resistance, porosity, capillary water absorption, color difference and surface strength. The results show that a calcium oxalate film with good protective performance can be successfully prepared without water retention in an environment with a relative humidity more than 76%. This is meaningful for the protective treatment of historic stones in environments with high humidity.