In order to address the spectrum fragmentation problem in flexible grid optical networks, the spectrum fusion oriented routing and spectrum allocation algorithm and spectrum defragmentation algorithm are proposed. The notions of spectrum allocation window, spectrum allocation point and spectrum fusion window are introduced to optimize the network resource utilization. To retain the spectrum consecutiveness as much as possible when establishing a lightpath and reduce the spectrum fragments, the proposed RSA algorithm considers the spectrum utilization in candidate links and their adjacent links. The spectrum defragmentation algorithm is used to make enough room for the blocked request by rerouting already established lightpaths. Simulation results reveal that the algorithms significantly improve the blocking probability performance and resource utilization under dynamic traffic in flexible grid optical networks.