Botnet is a kind of computer clusters which hackers have controlled by one to many command channels for malicious purposes. Using a botnet, hacker can conduct the denial of service attacks, send massive spam and steal confidential personal easily. A Botnet's control channel by the initial based on IRC protocol has become complicated channel based on HTTP. This paper introduces the control mechanism of HTTP-based botnet. In addition, this paper also analyzes a kind of botnet based on http protocol--Zeus. By setting up experimental environment, configuring Zeus tool and analyzing network traffic, we are able to understand the working mechanism of Zeus and the working principle of botnet based on HTTP protocol.