With the rapid development of information usage in society in the recent years, cloud systems are being more frequently used by Internet users. Among the many functions of cloud systems, the uploading and downloading files are more often used. But if the numbers of files are numerous, the keys which can decrypt the downloading files are also numerous. As a result, it will increase the loading of the cloud. Hence, some people use the key aggregate cryptosystem to solve the above situation by decreasing the numbers of the keys. However, it can't achieve the dynamic situation such as adding or deleting files in cloud environment, so we propose our scheme to solve it in addition to improve the key aggregate cryptosystem. We hope this study can be used as reference for other researchers that are interested in cloud file sharing systems, and that it can be further developed and applied to practical situations.