The invention discloses mannose oligomer oat vinegar and a preparation method thereof. The problems that in existing oat vinegar, nutritional ingredients are single, the mouthfeel and health-care functions are expected to be further improved, and a large quantity of konjac flying powder, waste yeast and red algae are abandoned and not effectively utilized are solved. According to the technical scheme, the preparation method comprises the steps that mixed slurry of the konjac flying powder, the waste yeast and the red algae is hydrolyzed through beta-mannosidase generated by aspergillus usamii and then mixed with oat slurry, and amylase and glucoamylase are added to obtain saccharified liquid; in the fermentation process, saccharomyces cerevisiae, aroma-producing yeast and acetobacter pasteurianus are added for further fermentation, and then the mannose oligomer oat vinegar is prepared. The preparation method is simple and low in production cost, the konjac flying powder, the waste yeast and the red algae are effectively utilized, the prepared oat vinegar is rich in nutritional ingredient and pure in flavor, the health-care effect of the mannose oligomer oat vinegar is enhanced, and the quality and health-care effect of the product are improved.