The invention aims at technology related to membrane separation technology used in sewage treatment, relates to a method and apparatus for measuring the diffusion coefficient of a cleaning agent in amembrane pollution layer, and belongs to the technical field of water treatment. The method comprises: measuring the thickness of a membrane-face pollution layer with a micrometer; downward fixing themembrane-face pollution layer on a measuring device; covering an upper surface (back side) of a membrane with chromogenic test paper used for marking the diffusion terminal point of a cleaning agent;injecting the cleaning agent to the apparatus till the cleaning agent reaches a preset scale and performing timing; and stopping timing till the color of the test paper is totally changed, and calculating the diffusion coefficient according to a diffusion model of the cleaning agent in the membrane pollution layer. According to the method, a diffusion model of a cleaning agent in a membrane pollution layer is established, so that quantitative monitoring of a cleaning agent diffusion process under a pressure in an experimental scale is achieved. The method is short in measuring time, high in repeatability, convenient to operate, and low in application cost, and provides efficient support to research of a cleaning agent diffusion mechanism in a chemical cleaning process.