The invention relates to a method for reducing viscosity of hydroxypropyl methyl cellulose by HCl. The method comprises the following steps: employing high-purity hydrochloric acid steam, introducingthe material in a HCl-resistant reactor, adding cellulose ether powder, under the temperature of 65-90 DEG C, and performing a degradation reaction to obtain the hydroxypropyl methyl cellulose. The high-purity hydrochloric acid steam is contacted with the cellulose ether powder through an upward flowing mode, the high-purity hydrochloric acid steam and the cellulose ether powder are uniformly contacted through the above contacting mode, the viscosity of hydroxypropyl methyl cellulose and uniformity can be effectively reduced, several micropores are arranged on a separator plate, leakage of cellulose ether can be guaranteed at a maximum degree, and the high-purity hydrochloric acid steam is upwardly flowed through a separator plate, so that contact of the high-purity hydrochloric acid steamand the cellulose ether powder can be effectively guaranteed.