This work develops a software-defined mechanism for dynamic bandwidth management, abbreviated to SDBM. This proposed SDBM mechanism was realized using the OpenFlow architecture and the OpenWrt platform. The resource allocation procedures and weighting policy were designed using SDN Controller to perform bandwidth management. This mechanism firstly monitors and analyzes networks and the statuses of applications and then, based on the results of the analysis, dynamically reallocates resource with software-defined discipline. Based on our simulation results, bandwidth utilization is improved and packet loss reduced by 12.4% and 8%, respectively.