Wi-Fi offloading is a technique for reducing 3G/3.5G/4G cellular network’s traffic load and users’ expense spent in 3G/3.5G/4G cellular network. Nevertheless, it needs to judge whether the offloading from the cellular network to the Wi-Fi network is valuable or not by considering the target Wi-Fi network’s situation. This paper proposed a Software Defined Network (SDN) -based method to pre-decide whether it is valuable to have offloading from the cellular network to the corresponding vehicle’s ahead 802.11p Wi-Fi RSU network or not. By utilizing the centralization architecture and the information of the current contexts, including speed, position and direction, of vehicles, SDN Controller can calculate whether the networking situation of the corresponding vehicle’s ahead 802.11p Wi-Fi RSU is good enough to offload or not before the vehicle enters into the signal coverage. The performance analysis shows that it can let vehicles have better networking situation and quality using our proposed scheme.