The invention relates to an electricity optical network routing spectrum allocation method and an electricity optical network routing spectrum allocation system, the method comprises the following steps: when an electricity service connection allocation request exists in an electricity optical network, acquiring a usable spectrum index on each link in the electricity optical network; according tothe usable spectrum index on each link, setting different spectrum window positions under different modulation formats and determining a candidate solution set of a service request which satisfies corresponding distance requirements; and selecting the modulation formats and routing spectrums, which meet a preset rule, from the candidate solution set to allocate. The routing modulation format spectrum allocation method can effectively reduce the needed total number of used spectrums in the network under premise of indirectly guaranteeing transmission quality and can improve spectrum use ratio.