A method for dynamic bandwidth allocation in Passive Optical Network (PON), said PON includes a OLT and a plurality of ONUs accessing to the OLT, comprising: classifying traffic which is to be communicated between the OLT and the ONUs into a plurality of service types, and granting a different priority to each type of the services; authorizing service ports of every type of services to transmit service data in descending sequence of said priorities of the services, and recording granting information of the service ports obtained from the authorization; reading out said granting information of every to-be-granted service port of a same ONU; and scheduling granted start time of data transmission of every to-be-granted port of current ONU, generating downlink granting messages including both said granting information and said granted start time of data transmission of every granted port of said current ONU, transmitting said downlink granting messages to said current ONU. This method for bandwidth allocation in the present invention can satisfy requirements of different types of services, increase bandwidth utilization ratio and realize equal bandwidth allocation.