The cause of Longyou Grotto has been analyzed by combining with historical materials.It has found through field survey that many grottos and rock caves have distributed on both banks of Qujiang River and its branches,and Longyou red sandstone grotto is not confined to Fenghuang Mountain.Based on the visit and investigation of local ancient villages,it has found that local vernacular architectures are pervasively constructed by red sandstone which has been consumed a lot.After the analysis and demonstration,it considers that red sandstone grottos are exploited as construction materials for local area,and rivers have been taken as the transport channel,convenient and cost-saving.