This application discloses a bandwidth allocation method, an optical line terminal (OLT), an optical network unit (ONU), and a system, where the method includes receiving a bandwidth request from each ONU, where the ONU includes an ONU1, generating a bandwidth map (BWMap) message according to bandwidth requested by the ONU and bandwidth configured for the ONU, where the BWMap message includes a first allocation identifier (Alloc-ID1), a first time corresponding to the Alloc-ID1, a second allocation identifier (Alloc-ID2), and a second time corresponding to the Alloc-ID2, and both the Alloc-ID1 and the Alloc-ID2 are allocated to the ONU1 for use, and sending the BWMap message to each ONU. Therefore, a problem that a transmission delay does not satisfy a requirement when a passive optical network (PON) system is applied to mobile backhaul is resolved, a data transmission rate and data transmission efficiency are improved, and user satisfaction is improved.