The invention discloses a CHA molecular sieve, a synthesis method thereof, and a denitration SCR catalyst synthesized by using the CHA molecular sieve. In the CHA molecular sieve, the molar ratio of silicon dioxide to aluminum oxide ranges from 5 to 75, the grain size ranges from 0.3 micrometer to 2 micrometers, after CHA molecular sieve raw powder is subjected to saturated steam treatment at thetemperature of 600 DEG C to 850 DEG C, tetra-coordinated aluminum accounts for more than or equal to 92 percent of the total aluminum amount, and the hexa-coordinate aluminum accounts for less than orequal to 8% of the total aluminum amount. The synthesis method comprises the step of synthesizing by using a composite organic template agent formed by a quaternary ammonium onium compound containingtetrahydronaphthalene or decahydronaphthalene groups and an N, N, N-dimethyl ethyl cyclohexyl quaternary ammonium onium compound. The molecular sieve disclosed by the invention has high hydrothermalstability under the condition of not having a large grain diameter, and the SCR catalyst disclosed by the invention also shows high nitrogen oxide reduction characteristics after being exposed at hightemperature and high humidity, and particularly shows high nitrogen oxide reduction characteristics in a temperature range of 200-550 DEG C.