The invention relates to a carboxyl nitrile rubber and a preparation method thereof. The carboxyl nitrile rubber is prepared from the following components: a carboxyl nitrile rubber raw rubber, a reinforcing agent, an antioxidant, an esterification vulcanization agent, an esterification dehydrating agent, and an esterification accelerant. According to the preparation method of the carboxyl nitrilerubber, mainly a polyhydric alcohol is used for vulcanizing the carboxyl nitrile rubber raw rubber, the alcohol group of the polyhydric alcohol reacts with the carboxyl group of the carboxyl nitrilerubber raw rubber to form an ester bond, a cross linking system is formed through the ester bond to complete the vulcanization of the carboxyl nitrile rubber raw rubber. According to the method, a vulcanization method different from that of the prior art is provided, and the method is a unique vulcanization method based on a condensation reaction.