The present invention provides a use of a high-temperature-resistant Cas protein, and a system and reagent kit for detecting a target nucleic acid molecule; specifically, the present invention provides a reaction system used for detecting a target nucleic acid molecule, said reaction system comprising: a guide RNA, Cas12b (formerly known as C2c1), and a nucleic acid probe; after a reaction is completed, detection of the nucleic acid is performed. In addition, by means of combining with nucleic acid amplification techniques (such as LAMP and the like), the sensitivity of the described detection method can be significantly improved. The detection system provided by the present invention can be used for rapidly detecting pathogenic microorganisms, gene mutations, single nucleotide polymorphisms, specific target DNA, and the like, and for quantifying nucleic acid samples.