The invention discloses a method for cleaning a plate-type heat exchanger and relates to an on-line cleaning method of the plate-type heat exchanger, in particular a cooling side of the plate-type heat exchanger in the production process of alumina. The method is characterized by comprising the following steps of: in the cleaning process, injecting a cleaning solution into the cooling side of the plate-type heat exchanger, and circularly cleaning; and after cleaning is finished, washing by using softened water to make the cleaning solution neutral. By adoption of the method, the in-place cleaning of the plate-type heat exchanger can be realized under the condition of non-disassembly of plate sheets, and trouble on disassembly can be avoided. The method is simple, time-saving, labor-saving, high in efficiency and low in cost; cleaning period is short; labor intensity is greatly reduced; and the damage to equipment body caused by manual cleaning is reduced.