The invention discloses a method for preparing a physochlaina-macrophylla water-binder pill. The method comprises the following steps: (1) smashing physochlaina macrophylla into coarse powder, extracting the coarse powder for 1-3 times by using a 70-95% (by volume) aqueous solution of ethanol through fluxing, combining extracts, filtering, recovering ethanol in a filtrate, and concentrating the extract at the temperature of 65-80 DEG C until the relative density is 1.03-1.3 to obtain a concentrated paste, wherein the aqueous solution of ethanol is 4-10 times the mass of the physochlaina macrophylla, and each time of extracting lasts for 1-4 hours; and (2) diluting the concentrated paste with purified water, adding aqueous ammonia, alkalifying the concentrated paste until the pH value is 8-11 to obtain alkalified diluted liquor, extracting the alkalified diluted liquor for 2-4 times to obtain an extract, recovering trichloromethane, volatilizing the extract by water bath until the extract is free from the smell of the trichloromethane to obtain a physochlaina-macrophylla extract, drying the physochlaina-macrophylla extract, smashing the physochlaina-macrophylla extract, feeding the smashed physochlaina-macrophylla extract into a pilling machine, and pilling to obtain the physochlaina-macrophylla water-binder pill, wherein the mass ratio of the concentrated paste to the purified water is 1:(0.8-1.5), and the trichloromethane is 0.5-3 times the volume of the alkalified diluted liquor. Experiments show that the physochlaina-macrophylla water-binder pill prepared by adopting the method disclosed by the invention has a good effect of relieving asthma.