Window-frame dam is a common measure in debris flow prevention engineering. The function of changing sediment size distribution is one of the main factors in the design of window-frame dam, and is also an urgent issue to be solved. In order to improve the design methods, we conduct a series of experiments to research the function of changing sediment size distribution of window-frame dam. Results show that 1) window-frame dam has a function of trapping coarse sand and discharging fine sand, and this function has a worst performance for viscous debris flow compared with trapping sub-viscous and intermediate debris flow; 2) we put forward the concept of critical particle size for the first time and give a variation range of critical particle; 3) the critical particle size and its variation range were affected by the density of debris flow and the type of window-frame dam (totally blocked, partially blocked, and open dams); 4) the critical particle size increases with the increase of density of debris flow for the same window-frame dam.