Pre-reservoirs are comparatively small reservoirs with an average water retention time of a few days.They are normally situated immediately above the larger main reservoirs whose inflowing water quality it is their purpose to improve.There are a number of physicochemical and biochemical processes within the pre-reservoirs,which hold back eutrophication in lakes.Numerical simulation was established in the paper and validated by the course of the rainstorm following after 25 hours,which shows that the flow field can help to make the water in pre-dam effuse and prevent the Dianchi Lake water from recharging.Soluble reactive phosphorus(SRP) eliminafion was calculated along the whole rainfall process with the simulation.The result shows that the simulation is identical to the calculated result and is able to account for SRP removal in pre-dams along the rainfall.SRP removal efficiency is over forty-eight percent in despite of rainstorm and so the pre-dam technique may be a rewarding choice for non point pollution control,especially for hypereutrophic lakes and reservoirs.So pre-dams may be a good and new technology for water purification in lakes for non-point pollution.