We present, a new framework for library-free peptide identification from data-independent acquisition (DIA) data, DIA-MS2pep, containing two main components: 1) MS2 spectrum self-demultiplexing and 2) large precursor mass tolerance database search. DIA-MS2pep empowers the discovery capability of DIA for both peptides and their modified forms. DIA data-specific library built with DIA-MS2pep outperforms sample-specific DDA data library in accuracy and reproducibility of protein quantification, with the merit of discovering protein modification.