The invention relates to a method for producing hydroxypropyl methylcellulose. The method comprises the following steps: 1) performing pretreatment, namely preparing 60-100-mesh cotton powder from purified cotton; 2) alkalifying, namely reacting the powder with alkali to produce alkali cellulose in a mixed solvent; 3) etherification, namely carrying out an etherification reaction among the alkali cellulose, epoxypropane and chloromethane; 4) neutralizing, namely neutralizing the residual alkali with an acid; 5) desolventizing, namely discharging the mixed system into a desolventizing kettle, adding hot water to the top while stirring, introducing water vapor from the bottom of the desolventizing kettle so as to form a solid-liquid-gas three phase; discharging and recovering most of the liquid solvent from a pipeline in the middle of the desolventizing kettle; and 6) performing post-treatment, namely finally centrifuging, granulating, drying, packaging, thereby obtaining the product. According to the method disclosed by the invention, since the recovered reactants participate in the separation process due to the added hot water, compared with the hydroxypropyl methylcellulose (HPMC) prepared by the conventional technical method, the product prepared by the method disclosed by the invention has the advantages that the purity of the crude product is high, and the treatment difficulty of the HPMC at the later stage is reduced.