The granularity of the flexible bandwidth optical network is the spectral slots,which is much smaller than that of the wavelength switch optical network.For the dynamic clients’ connections setup and tear down processes,it will give rise to fragmentation of spectral resources.It is the decline in the probability of finding sufficient contiguous spectrum for new connections that result in the fragmentation of spectral resource.To be more specific,these spectra may be unavailable and waste.In this case,the severe waste of the spectrum will lead to low efficiency in spectral utilization and will not adapt to large capacity requirements of transmission in the future.Because path computation element(PCE) framework has the characteristics of the central disposal and deployment of the spectrum resource,we construct the spectral resource allocation scenario based on PCE framework in the flexible bandwidth optical network to use spectrum resource effectively.Based on the principle of the generation of the fragmentation,we put forward a spectrum resource defragmentation algorithm to consolidate the available spectrum for clients’ connections.The simulation results indicate that this algorithm is able to reduce fragmentation of network,improve the continuity of spectral resource,reduce the blocking rate of services in the network and improve the spectral efficiency significantly. The granularity of the flexible bandwidth optical network is the spectral slots, which is much smaller than that of the wavelength switch optical network. For the dynamic clients’ connections setup and tear down processes, it will give rise to fragmentation of spectral resources. It is the decline in the probability of finding sufficient contiguous spectrum for new connections that result in the fragmentation of spectral resource.To be more specific, these spectra may be unavailable and waste.In this case, the severe waste of the spectrum will lead to low efficiency in spectral utilization and will not adapt to large capacity requirements of transmission in the future.Because path computation element (PCE) framework has the characteristics of the central disposal and deployment of the spectrum resource, we construct the spectral resource allocation scenario based on PCE framework in the flexible bandwidth optical network to use spectrum resource effectively. Based on the principle of the genera tion of the fragmentation, we put forward a spectrum resource defragmentation algorithm to consolidate the available spectrum for clients’ connections. The simulation results indicate that this algorithm is able to reduce fragmentation of network, improve the continuity of spectral resource, reduce the blocking rate of services in the network and improve the spectral efficiency significantly.