Aiming at the problem that the existing defragmentation methods in elastic optical networks can not make full use of the maximum free spectrum block and the number of service migrations is large, a defragmentation algorithm with maximum bandwidth service precise migration triggered by residual duration(BBSEM-DF) is proposed. Firstly, a fragment measurement model is constructed to measure the distribution of spectrum fragments in the link. Then, the threshold is set according to the amount of fragments and the remaining duration of business to trigger the defragmentation process. Finally, the maximum continuous service group is migrated to the free spectrum block in the link that matches the required number of frequency slots for accurate migration. The simulation results show that compared with the traditional defragmentation algorithm, this algorithm can reduce the spectrum fragments, reduce the blocking rate, and improve the network performance.