Abstract: Based on experimental data at the Arkesu water balance experimental station in 1984 to 1987, the variation laws of phreatic evaporation were analysed, the empirical formulae for calculating the phreatic evaporation were presented. The results show that C-H Type Formula suposes that the phreatic evaporation is proportional to the atmosphere evaporation capacity. But in fact, as for a given phreatic depth, when the atmosphere evaporation capacity is small enough, the phreatic evaporation increases with the atmosphere evaporation capacity, after the atmosphere evaporation capacity reaching the critical atmosphere evaporation capacity, the phreatic evaporation is close to a constant, limited phreatic evaporation E_max. It is difficult to determine the parameter of present formula given by Tsinghua's scholar, and its parameter η decreases with the phreatic water depth, and it is not a constant. For the new empirical formula, its construction is reasonable, its conception is explicit, and its calculation is easier than the former formula.