The invention relates to a dynamic resource allocation method of the optical network, and provides a dynamic routing and spectrum allocation method based on the non-interval degree of a frequency spectrum, and the routing and spectrum allocation method solves the problem that as spectrum fragments are generated by dynamic connection and disconnection in an existing dynamic RSA, the blocking rate of the network increases. The method comprises the steps that one or more paths are pre-selected between a source node and a target node in the network to serve as alternative routes; the frequency gap of each link through which each alternative route passes is counted, and a collection of idle frequency gaps on the links is found; the frequency gas collection is grouped according to the adjacent relation, and alternative frequency spectrum blocks are formed; for each route communicated with the corresponding the alternative frequency spectrum block, the non-internal degree of the path frequency spectrum is calculated; the corresponding route and the alternative frequency spectrum block when the non-internal degree of the path frequency spectrum is the maximum are selected; one or more frequency gaps are distributed according to the demand of service bandwidth. In the route building process, the method can effectively utilize the frequency spectrum fragments caused by dynamic connection and disconnection, and therefore the network performance is optimized.