Abstract: In the multi-spot-beam satellite communication system, the efficiency of resources is constrained by on-board energy. It is important to improve the efficiency of resources. Taking the free space path loss, the antenna gain, the rain attenuation and inter-beam interference of each beam into account, a joint power and bandwidth allocation algorithm based on different traffic demands and channel conditions over satellite downlinks is proposed, which increases the flexibility of the multi-spot-beam resource optimization. The algorithm adopts a two order differential object function and obtains a lower bound of the optimal power and bandwidth with Lagrangian duality theory and sub-gradient method. The simulation results show, compared with separate power and bandwidth optimization algorithm, the joint optimization algorithm can allocate system capacity more flexible and satisfy different traffic demands and channel conditions. So it improves on-board resource utilization efficiency.