The invention provides a spectrum allocation method and system. The spectrum allocation method comprises the steps of: based on a source node and sink nodes of services, constructing a small-particle-size service group; constructing a continuous spectrum plane for the small-particle-size service group; and according to the continuous spectrum plane, allocating spectrum resources. According to theinvention, the small-particle-size service group is constructed by carrying out classification on the services, the services with path similarity are partitioned into the same group to be subjected touniform routing and spectrum allocation so as to reduce complexity of service bearing, and continuous spectrum allocation is carried out on the basis of the small-particle-size service group so as toeffectively reduce generation of spectrum fragments and improve spectrum availability of an elastic optical network.