Security vendors are facing a serious problem of defeating the complexity of malwares. With the popularity and the variety of zeroday malware over the Internet, generating their signatures for detecting via anti-virus (AV) scan engines becomes an important reactive security function. However, AV security products consume much of the PC memory and resources due to their large signature files. AV cloud computing becomes a popular solution for this problem. In this paper, a novel Automatic Malware Signature Discovery System for AV cloud (AMSDS) is proposed to generate malware signatures from both static and dynamic aspects. Our experiments on millions-scale samples suggest that AMSDS outperforms most state-of-the-art automatic signature generation techniques of both industry and academia. Key words: anti-virus, network security, malware, cloud computing