The invention discloses a fermentation dairy product and a preparation method thereof. The preparation method comprises the following steps of: (1) mixing components in a milk raw material, homogenizing, sterilizing and cooling, wherein the milk raw material comprises animal milk and a thickening agent; (2) adding mesophilic lactic acid bacteria under the sterile condition, filling in a sterile filling manner, and fermenting in a fermentation warehouse at the temperature of 20-37 DEG C; and (3) firstly preheating the milk product obtained by fermentation in step (2), then sterilizing for 4s to 10 minutes at the temperature of 60-120 DEG C, and storing, thus the fermentation dairy product is obtained. By adopting the preparation method disclosed by the invention, time that a fermentation product occupies a tank can be effectively reduced, and a phenomenon of false package expansion can be avoided; and a quality guarantee period of the fermentation product is long, and taste is good.