To overcome the disadvantage of high consumption of energy and bandwidth when the existing cloud storage technology which is applied to the smartphone, based on the Dropbox platform as a cloud service provider, an efficient and safe File Management system based on Cloud Instances (FM-CI) was designed. FM-CI supported download, compress, encrypt, convert operations, and file transfer between two smartphone users' cloud storage spaces. In addition, due to frequent open cloud instances may still increase the cost of the user, a protocol for users to share their idle instances and a file transfer scheme for sharing instance was designed. Simulation results show that, FM-CI can efficiently complete the file operations with less time and bandwidth, and the performance of FM-CI is better than those of the latest cloud storage schemes.