Abstract: Security has become an important factor restricting the development of cloud computing.This paper analyzed the cloud security objectives from 6 aspects:service constancy,service authenticity,data integrity,data confidentiality,data availability and non repudiation.It summed up the cloud computing risk in seven categories:physical security risk,computing security risk,trusted computing security risk,network security risk,management security risk,storage securi-ty risk and application security risk,and elaborated the corresponding security strategies for every security risk.The stronger the securityis,the greater the consumption of computing,memory,and bandwidth resourcesis.This paper provided on-demand security framework for cloud computing which uses different safety protection measurement according to service type,security level and access network risk.The advantage of the framework was analyzed and the application method was provided.

Key words: Cloud computing,Cloud model,Cloud security,Cloud security framework,Cloud security objective,Access control