<p>Fission products are released from the core as aerosols during severe nuclear reactor accidents. Most aerosols dispersed in containment will be deposited on the floor and could be washed by condensate flow with the migration of radioactivity. Investigating the aerosol wash-down process is interesting for evaluating radioactivity in nuclear power plant containment. THAI-AW3-LAB experiment was carried out by Becquerel to study the wash-down process of insoluble aerosols on walls, based on which parameters such as the wash-down rate and flow coverage were obtained. However, injecting water flow in the investigation simulated the condensation process on a plate, and experimental verification of the natural aerosol wash-down process on walls of a condensation environment needs to be improved. To further explore the aerosol erosion and wash functions on containment floor, an experiment on aerosol wash-down characteristics of a horizontal plate in condensing environment is carried out in this paper. Experimental visualization records the complete process of aerosol erosion and wash-down, focusing on how aerosols move with condensate in environments with different deposition densities of aerosols. The washing rate and share of aerosols under different deposition environments are discussed. Experimental results show that wash-down process may have three different stages with time. This paper describes a complete process in which aerosols on a horizontal plate are washed by the growing or flowing condensate. For example, a considerable part of deposited aerosols can quickly enter the condensate and move with condensate in an early stage of the growth of the droplets. Still, the flowing condensate may hardly erode the remaining aerosol on the plate. The washing rate varies with the washing process and shows a long-term exponential washing process lasting for several hours.</p>