The disclosure claims a method and system for reporting a buffer size index; the method includes: presetting a first table which is a buffer size index table used by a Long Term Evolution (LTE) system, and presetting a second table which is a buffer size index table by increasing a maximum buffer size value to Bmax-A and increasing a minimum buffer size value to Bmin-A based on the first table, or by increasing a maximum buffer size value to Bmax-A, and increasing a number of the buffer size index to NLTE-A; choosing, by a User Equipment (UE) in reporting a Buffer State Report (BSR), to inquire on the preset first table or second table for the buffer size index; and sending, by the UE, the inquired buffer size index to a base station in the BSR. By adopting the method and system according to the disclosure, the scheduling efficiency in carrier aggregation is improved effectively and reasonable resource allocation is ensured.