The invention relates to a method and a device for dynamic bandwidth allocation in gigabit passive optical network, wherein the method comprises the steps: at first, predicating the bandwidth of current Alloc-ID, regarding the predication value as input speed of a token bucket; then computing DBA bandwidth required by the current Alloc-ID and comparing the DBA bandwidth with a current token number of the token bucket, if the DBA bandwidth is smaller than the token number, allocating relative bandwidth to the current Alloc-ID and writing the allocated bandwidth into a BWMAP table; and sending the BWMAP table to an optical network unit; the device comprises a T-CONT flow policy table, a flow statistic module, a DBRu table module, a bandwidth predication module, a token bucket flow control module, a bandwidth allocation module, a BWMAP table configuration module, a BWMAP table sending module and a scheduling cycle control module. The method and the device enhance the response speed for the bandwidth allocation of T-CONT.