The present invention provides a method for accessing a system memory, wherein the method includes the steps of: reading a descriptor from the system memory, where the descriptor includes a buffer start address field and a buffer size field, wherein the buffer start address field includes a start address of a buffer in the system memory, and the buffer size field indicates a size of the buffer; receiving multiple packets, and writing the multiple packets in to the buffer; modifying the descriptor according to the multiple packets stored in the buffer to generate a modified descriptor, wherein the modified descriptor only comprises information of part of the multiple packets or does not comprise information of any one of the multiple packets; and writing the modified descriptor into the system memory.