A method and system for decompressing a compressed file is disclosed in this invention, the method comprising: reading a compressed data block from the compressed file; decompressing the compressed data block; outputting the decompressed data for storage into a decompressed file; deleting the compressed data block that was decompressed from the compressed file. The proposed method and system in this invention can reduce the unnecessary repeated data between compressed data and decompressed data. The storage space requirement will be reduced during decompression, and the existing compression/decompression algorithms need not be changed by using this invention. Thus, this invention is easy to be integrated into existing compression/decompression tools.