In this paper, a critical pattern selection method for full-chip source mask optimization is proposed. The critical frequency of the pattern represents the characteristics of the pattern. The location and contour information of critical frequency are used to describe the distribution characteristics of the critical frequency in the frequency domain. The corresponding critical frequency extraction method, covering rules, grouping method, and critical pattern selection method are designed to realize the critical pattern selection of the full-chip source mask optimization. Commercial computational lithography software Tachyon of ASML company is used for simulation verification. The comparison with the similar technique of ASML company shows that the process window obtained by this method is better than the ASML Tachyon method.