The invention discloses a method for synthesizing nucleic acid under a constant temperature condition, a kit and application. The method comprises the steps that a nucleic acid is provided, the nucleic acid is sequentially composed of an M1c region, an F1c region, an M region, an R1 region and an M2c region in the 3'to 5 'direction, the M region comprises an M1 region and an M2 region, and annealing of the M2c region at the 5'end and the M1c region at the 3' end of the nucleic acid and the M region on the same chain can form a closed ring structure; a first oligonucleotide I and a second oligonucleotide II of a primer are respectively annealed with an F1c region and an R1c region of the nucleic acid, and the nucleic acid is used as a template to react so that a nucleic acid chain continuously extends. According to the nucleic acid synthesis method disclosed by the invention, the length of the core primer is about 30bp and is shortened by 10bp compared with the core primer which is about 40bp and is used by LAMP (Loop-Mediated Isothermal Amplification) and CAMP (Cyclic Amplified Polymorphism), so that about 1/4 of primer cost can be saved, and meanwhile, the reaction rate of nucleic acid synthesis is also improved.