The invention, which relates to a optical network communication technology field, discloses a frequency spectrum resource distribution method in an elastic optical network and a system thereof. The method comprises the following steps: S1. when a client request arrives, selecting one optical path with maximum available frequency gaps from candidate optical paths; S2. if a bit error rate needed by the client request is less than a preset bit error rate threshold, selecting a low order modulation mode, otherwise executing a step S3; S3. if a hop count of the optical path exceeds a preset hop count threshold, selecting a low order modulation mode, otherwise selecting a high order modulation mode; S4. determining whether there are enough continuous available frequency gaps, if there are enough continuous available frequency gaps, executing a step S5, otherwise obstruct; S5. selecting the frequency gap from a minimum label and establishing the optical path according to the selected modulation mode. According to the invention, during the frequency spectrum resource distribution, an error rate requirement and the hop count are considered. When the resources are sufficient, the information transmission quality requirement can be considered too. The frequency spectrum can be saved and the bit error rate is not increased.