In this paper, a novel Automatic Malware Signature Discovery System for AV cloud is proposed to generate malware signatures from both static and dynamic aspects. It keeps a good workload balance between the desktop and cloud services. It can automatically generate a lightweight signature database with the size hundreds times smaller than traditional signature ones. In the AV cloud model, users do not need to install a large virus signature file, but a lightweight set of cloud signatures. The benefits include easy deployment, low costs of operation, and fast signature updating. Further, Automatic Malware Signature Discovery System signatures can be easily integrated into existing AV products.