The rapid growth of wireless networks makes cognitive coexistence an important technology to use the spectrum efficiently. In this paper, a novel sensing based delay optimal method is considered to meet the requirement of the delay-sensitive traffics as well as to minimize the collision between the primary user and the secondary user. After establishing the Markov Decision Process (MDP) model, we proposed a spectrum access policy using EXP-LOG rule, in which the transmission probability is determined by the sensing result and queue state information. The simulation results verify the validity and feasibility of the proposed policy.