Salt weathering is one of the important reasons of weathering deterioration of stone cultural relics, but its internal mechanism and process need to be further studied. Methods: In this paper, we collected the core and weathered samples of Buddha Grottoes from Niujiaozhai of Renshou to do the study. Their mineral and chemical compositions were compared and researched, the composition and crystallization state of soluble salt in samples are Focused. Results: Researches show that (i) the weathered samples have 16 times higher contents of SO3 than the core samples; (ii) the core samples were detected to have trace amounts of gypsum only in the 0~22 cm surface, whereas the weathered samples contain 17. 5 times higher content of gypsum, and a few Na2SO4 and NaCl. This indicates that the weathering deterioration of grottoes is related to soluble salt, especially sulfate. Further studies on the properties of Na2SO4, NaCl and gypsum show that Na2SO4 can undergo repeated crystallization reaction of Na2SO4+H2O Na2SO4·10H2O in response to climatic conditions, the volume expansion during crystallization is 416%, especially in humid climate, the secondary crystallization of Na2SO4 can cause more damage to rocks. NaCl can reduce the phase transition temperature (to 17.6℃) and solubility of Na2SO4, and amplify its role in the process of rock weathering. CaSO4 has slight effect on rock weathering, but it can be used as an indicator for the accumulation of soluble salts. Combined with the studies on the temperature and humidity monitoring data of Renshou Giant Buddha, it was found that the changes of daily temperature and humidity with time naturally fit the transition conditions of Na2SO4—NaCl system.