A bandwidth assignment method, wherein the method comprises: setting, by an optical line terminal, a maximum bandwidth grant size and a maximum burst bandwidth grant size for an optical network unit; receiving, by the optical line terminal, a bandwidth assignment request of the optical network unit; and when the optical line terminal determines, according to the bandwidth assignment request, that a bandwidth grant size requested to be assigned in the bandwidth assignment request is greater than the set maximum bandwidth grant size and less than or equal to the set maximum burst bandwidth grant size, delivering, by the optical line terminal in response to the request, the bandwidth grant size requested in the bandwidth assignment request of the optical network unit to the optical network unit, wherein, (a), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a byte count requested to be assigned in the bandwidth request, the maximum bandwidth grant size is a maximum bandwidth grant byte count and the maximum burst bandwidth grant size is a maximum burst bandwidth grant byte count; or, (b), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a multiple of a bandwidth grant byte count assigned within a unit grant period, the maximum bandwidth grant size is a multiple of a bandwidth grant byte count assigned within a unit grant period and the maximum burst bandwidth grant size is a multiple of a bandwidth grant byte count assigned within a unit grant period, wherein a value of a multiple corresponding to the bandwidth grant size requested to be assigned is different from a value of a multiple corresponding to the maximum bandwidth grant size and, wherein a value of a multiple corresponding to the maximum burst bandwidth grant size is different from a value of a multiple corresponding to the bandwidth grant size requested to be assigned and a value of a multiple corresponding to the maximum bandwidth grant size; or, (c), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a time requested to be assigned in the bandwidth assignment request, the maximum bandwidth grant size is a maximum bandwidth grant time and the maximum burst bandwidth grant size is a maximum burst bandwidth grant time. setting, by an optical line terminal, a maximum bandwidth grant size and a maximum burst bandwidth grant size for an optical network unit; receiving, by the optical line terminal, a bandwidth assignment request of the optical network unit; and when the optical line terminal determines, according to the bandwidth assignment request, that a bandwidth grant size requested to be assigned in the bandwidth assignment request is greater than the set maximum bandwidth grant size and less than or equal to the set maximum burst bandwidth grant size, delivering, by the optical line terminal in response to the request, the bandwidth grant size requested in the bandwidth assignment request of the optical network unit to the optical network unit, wherein, (a), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a byte count requested to be assigned in the bandwidth request, the maximum bandwidth grant size is a maximum bandwidth grant byte count and the maximum burst bandwidth grant size is a maximum burst bandwidth grant byte count; or, (b), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a multiple of a bandwidth grant byte count assigned within a unit grant period, the maximum bandwidth grant size is a multiple of a bandwidth grant byte count assigned within a unit grant period and the maximum burst bandwidth grant size is a multiple of a bandwidth grant byte count assigned within a unit grant period, wherein a value of a multiple corresponding to the bandwidth grant size requested to be assigned is different from a value of a multiple corresponding to the maximum bandwidth grant size and, wherein a value of a multiple corresponding to the maximum burst bandwidth grant size is different from a value of a multiple corresponding to the bandwidth grant size requested to be assigned and a value of a multiple corresponding to the maximum bandwidth grant size; or, (c), the bandwidth grant size requested to be assigned in the bandwidth assignment request is a time requested to be assigned in the bandwidth assignment request, the maximum bandwidth grant size is a maximum bandwidth grant time and the maximum burst bandwidth grant size is a maximum burst bandwidth grant time.