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Deterministic Finite Automaton (DFA) is widely employed in performing fast regular expression matching for numerous network applications. However, constructing DFA recognizing a set of regular expressions is time-consuming especially when the set is huge. In this paper, an improved approach is introduced to accelerate DFA construction based on the hierarchical merging method. The experiments demonstrate that the improved approach proposed in this paper performs about 1.6 to 3.4 times faster than the original hierarchical merging method, and 120 to 1500 times faster than the classical subset construction algorithm.