According to the special structure of the unconfined water in the subsurface evaporation-filtration ex-perimental system,the difference between the precipitation filtrate recharging to groundwater and the natural unconfined water as well as its error in the precipitation filtrate recharging monitoring is analyzed.Based on the mechanism of precipitation filtrate recharging,a treating way for using the gravity water storage volume regulat-ing in the unsaturated zone and the calculation of soil re-storage to the data on the subsurface evaporation-filtration experimental system has been proposed.That is the method of P ' r - W-P r ,which is simple in structure and fine in concept.Tried with data from Taigu Water Balance Experimental Field,it shows that the way is fit to the natural conditions.The method of P ' r - W-P r can realize the transform of the data from the subsurface evaporation-filtration experimental system to the natural precipitation filtrate recharge calculation.It can also mend up the shortage of its monitoring to the recharge.It leads experimental data close to the natural ones.