The invention provides a preparation method of Monostroma nitidum crude polysaccharide and an application thereof in a cigarette. The invention is characterized in that the preparation method of the Monostroma nitidum crude polysaccharide comprises the following steps: taking Monostroma nitidum, inactivating enzyme at high temperature, washing to remove impurities, drying and grinding, adding water to extract, performing solid-liquid separation, concentrating under reduced pressure, precipitating with ethanol, and washing and drying to obtain the crude polysaccharide. The application of the crude polysaccharide is as follows: the Monostroma nitidum crude polysaccharide is dissolved in water to prepare material solutions of different concentrations; and the material solutions are sprayed to cut tobacco evenly. The sensory evaluating result shows that after the Monostroma nitidum crude polysaccharide is added in the cigarette, the smoke of the cigarette has no peculiar smell, the quality of aroma is improved, the non-subject odor and irritation are reduced, the remaining aroma is sweet and refreshing, and the overall smoking quality is increased. In the preparation method, the original performances of the cigarette are not changed, the original processing technology is not changed, the aroma quality of the cigarette is improved, and the sensory comfort level of the cigarette is increased, thus the method has higher meaning and value in actual production.