HTTP Distributed Denial of Service (DDoS) flooding attack aims to deplete the connection resources of a targeted web server by transmitting a massive amount of HTTP request packets using botnets. This type of attack seriously deteriorates the service quality of the web server by tying up its connection resources and uselessly holds up lots of network resources like link capacity and switching capability. This paper proposes a defense method for mitigating HTTP DDoS flooding attack based on software-defined networking (SDN). It is demonstrated in this paper that the proposed method can effectively defend the web server and preserve network resources against HTTP DDoS flooding attacks.