The initial resource allocation for dynamic service requirement may lead to frequency spectrum fragment in Elastic Optical Networks（EON）

which will influence the allocation and resource efficiency of subsequent services.Spectrum fragment is one of the core problems for Routing and Spectrum Allocation（RSA）.This paper proposes an improved RSA algorithm for EON based on weights evaluation of spectrum fragments.This improved algorithm considers the Dispersion of Fragmentation（DF）as new parameter for evaluating spectrum fragments.Theoretical analysis and numerical simulation results show that compared with traditional RSA algorithm

the proposed algorithm can reduce the spectrum fragments degree and enhance the using rate of spectrum resources.