An automatic software audit system includes a client and a server. The client includes a network interface, a software installation record database, a software audit rule database, a software release database and a central processing unit (CPU). The network interface is coupled to the client. The software installation record databases stores a software installation record of the client. The software audit rule database stores a software audit rule. The software release database stores a software release record of the client. The CPU installs an agent program to the client to collect the software installation record, and generates a software audit result of the client according to the software installation record, the software audio rule and the software release record.