The invention discloses a method for extracting tobacco polysaccharides from waste tobacco stems in farmlands. A flash extraction technology is utilized, and the method comprises the steps of pretreating; decolorizing and degreasing; extracting tobacco polysaccharides; removing proteins by using a Sevage method; carrying out after-treatment comprising the technical means such as alcohol precipitation, centrifuging, washing and drying; and extracting to obtain the tobacco polysaccharides with the purity of 92%. The method disclosed by the invention is high in extraction speed, mild in extraction condition, high in extraction efficiency, simple in operation and capable of easily realizing industrial production; compared with a common plant polysaccharide reflux extraction method, the method disclosed by the invention is short in extraction time and capable of reducing the energy consumption, so that the production cost is reduced; in addition, the invention provides a reference method for utilizing waste tobacco stems in farmlands and developing corresponding products.