A bandwidth allocation method and a related device are disclosed. An exemplary method includes: when a total sum of a cumulative sum of fixed bandwidth configuration upper limits of traffic bearing entities and a cumulative sum of assured bandwidth configuration upper limits of the traffic bearing entities is greater than a maximum bandwidth value of a passive optical network (PON) port of a piece of central office equipment, determining, by the central office equipment based on a bandwidth configuration upper limit of each traffic bearing entity and a required bandwidth value of the traffic bearing entity, a bandwidth value actually allocated to the traffic bearing entity. In this way, a part of bandwidth is allocated to each traffic bearing entity. This avoids a case in which no bandwidth is allocated to some traffic bearing entities. Therefore, resource allocation is more appropriate.