The invention provides a method for automatically expanding defect pattern library. The method comprises the following steps: (1) preparing defect pattern initial data for representing an initial defect pattern; (2) evolving the critical dimensions of the initial defect pattern in batches by making a part of the initial defect pattern move in relative to the rest of the initial defect pattern and/or changing the critical dimensions of at least a part of the initial defect pattern so as to automatically generate a virtual defect pattern database; (3) simulating process bias of the virtual defect patterns, and selecting new defect patterns according to the process bias simulation results; (4) adding the new defect pattern to the defect pattern library to form new defect pattern data so as to expand the defect pattern library.