In the era of cloud computing, it is convenient to share large-scale data among various kinds of users. As a kind of attribute-based encryption, ciphertext-policy attribute-based encryption (CP-ABE) is a potential technique for realizing fine-grained access control on shared data. However, traditional CP-ABE is not suitable for mobile cloud computing, where mobile users are resource-limited and privacy is fragile. In this paper, we propose an efficient and privacy-aware attribute-based data sharing system supporting offline key generation and offline encryption. In the proposed system, sensitive attribute values specified in an access structure are not explicitly sent along with a ciphertext. The online/offline encryption mechanism alleviates the computational burden of mobile users by performing most of encryption tasks without draining the battery. In addition, the online/offline key generation mechanism allows the attribute authority to finish most of operations in the key generation process in advance, which enables efficient mobile user registration. Finally, the proposed system is proven fully secure in the standard model and performance analysis shows its effectiveness in mobile cloud computing.