This application provides a method for reporting a buffer status report and a communications apparatus. The method includes: obtaining, by a terminal device, a size of a first uplink resource; determining, by the terminal device based on the size of the first uplink resource, that a priority of an uplink buffer status report (UL BSR) is higher than a priority of an SL BSR, or that a priority of a sidelink buffer status report (SL BSR) is higher than a priority of a UL BSR.