In order to improve the throughput of multichannel CR, a joint allocation of spectrum sensing time and subchannel sensing threshold based on the alternating direction optimization is proposed. Based on the SU's listen-before-transmit frame structure, an optimized allocation model is built to maximize the aggregate throughput of the SU over all the subchannels, providing that the communication demand of the PU and the performance of the subchannel spectrum sensing are guaranteed. The joint optimization algorithm is proposed to obtain the optimal solutions to the model through alternately optimizing sensing time and threshold. The simulation results show that there exist the optimal sensing time and threshold that maximize the SU's throughput, and compared to the previous schemes, the joint allocation can improve the SU's throughput.