Disturbance usually causes fragmentation and degradation of biological soil crusts (biocrusts). To clarify the mechanism of biocrust degradation caused by disturbance-induced fragmentation, we selected the moss (Didymodon vinealis) dominated biocrusts (moss crusts hereafter) from the hilly-gully region of the Loess Plateau as the object of study. The dehydration rates and contents of osmoregulatory substances, malondialdehyde and photosynthetic pigment in the individual mosses and mosses of the moss crust patches with different sizes, which measured in the diameters of 1 cm, 2 cm, 3 cm, 4 cm, and 5 cm, were measured under 25 rounds repeated de-rehydration conditions, so as to reveal the biological mechanism of biocrusts degradation after disturbance. The results showed that (1) the dehydration rate of mosses increased with the decrease of patch sizes of moss crusts. The dehydration rate of mosses in the patch with diameter of 1 cm was twice of that with diameter of 5 cm. (2) The contents of soluble sugars, soluble proteins, and chlorophyll of mosses in the patches with diameter less than 5 cm were lower than those with diameter of 5 cm patches, and the content of malondialdehyde had no significant change with different moss crust patches sizes after 25 days of repeated de-rehydration. (3) Dehydration rate of mosses was extremely significantly negatively correlated to sizes of moss crust patches, the contents of soluble sugars, soluble proteins, and chlorophyll. The results of the study showed that the sizes of moss crust patches affected the dehydration rate of mosses in the patches and then affected their osmotic regulation and photosynthetic capacity. The dehydration rate of mosses increased as the size of moss crust patches decreased, which damaged to physiological activities of mosses and might be the cause of degeneration induced by fragmentation of moss crust patches. This study explains the physiological causes of mosses degradation after disturbance from the perspective of mosses physiology, which provides scientific basis for the protection and management of biocrusts.