The invention relates to the technical field of semiconductors and discloses a critical dimension shrink method. According to the invention, CO is adopted to replace the traditional fluorocarbon compounds and H-containing fluorocarbon gases, and serves as a high molecular polymer gas in the critical dimension shrink method to support plasma etching under low-frequency radio-frequency power, so as to effectively control the molecular chain length and plasma energy and density in the critical dimension shrink process. The method can control the molecular chain lengths of polymers resulting from plasma etching and avoid the formation of long molecular chain polymers during the plasma etching process. Besides, the plasma etching process of the critical dimension shrink method is carried out under a high pressure environment, to further improve the phenomenon of physical bombardment of polymer deposition during the etching process, so as to effectively prevent the formation of stripes and improve the line edge roughness of the etched structure resulting from critical dimension shrink. The critical dimension shrink method can further improve the process quality and ensure the effective and high-quality shrink of critical dimension.