The paper presents a new theory of space mathematicalbase of large－scale GIS and digital earth. In order to improve scientific decision－making and agility of flood prevention and the construction efficiency of flood prevention information system based on GIS,the flood prevention information system of Xianning City based on the new theory was developed. The system employed the vector－grid coalescence technology with DRG as the context reference and followed the technical route that the consumer can label the symbol of special entity at any time. The system offered an advanced technology for the flood prevention of Xianning City, obtained satisfactory results,demonstrated that the theory and technical route mentioned above have good prospects for carrying into execution. 