Aimed at the problem that energy efficiency of spectrum sensing is low in cognitive radio (CR) system, a joint optimization algorithm based on bisection and iteration is proposed. This algorithm sharply raises the energy efficiency of CR system by jointing transmitting power and sensing slot to reach the optimization. And in the process of optimizing parameter ξ, the methods of bisection and iteration are jointed to reach the optimization. Through theoretical analysis, compared to other algorithms, this algorithm is faster at optimization speed, lower in complexity, faster at function convergence speed and larger in space for extension in application. The results of numerical simulation show that under condition of provided detection performance, the proposed algorithm can deal with the optimization of energy efficiency well. Under conditions of the constant detection probability of P^d=0.9 and P^d=0.7, energy consumption of the proposed algorithm is lower than that of others, and the algorithm in system throughput is superior to others. This provides a theory support to CR system development, and is more in line with the energy-saving trend for CR system development in the future.