Relying on ofloading computation tasks to the network edge, ultra dense networks (UDNs) are capable of providing delay-aware service to nearby users. Meanwhile, software defined networking (SDN) is deemed as an effective technology to ease the management of infrastructure plane and control plane in UDNs. Specifically, the centralized SDN controller is capable of managing the whole network globally. With the increasing demands for various applications as well as the limitation of computation, storage and communication resource, how to allocate spectrum resource appropriately is imperative. In this paper, we mainly show solicitude for spectrum sharing and edge computation ofloading problems in SDN-based ultra dense networks. To address this issue, we propose a second-price auction scheme for ensuring the fair bidding for spectrum rent. Moreover, the MBS edge cloud is termed as the buyer, and the SBS edge clouds are the sellers who sell the ofloading resource to the MBS edge cloud. To be specific, the spectrum sharing and computation ofloading scheme is executed in the SDN controller, and the controller is responsible for distributing spectrum allocation instructions to the infrastructure plane. Finally, experimental results validate the effectiveness of our proposed scheme in SDN-based ultra dense networks.