By analyzing the impact of precipitation on the crystallization of brine, a conclusion is obtained that the precipitation leading to brine desalination, salt dissolving and pool damage is progressively increased. The precipitation leading to unfreezing salts and the pool damage is much larger than that of plastic film cover. Therefore, the decision makers can consider the plastic film cover precipitation only. By using equivalence principia, the major factors which influence decisionmaking of plastic film cover to precipitation are analyzed. Results show that the influences of plastic film cover cost, rime cistern acreage and changes of salt prices are less, but the plastic film cover precipitation has a positive correlation to the plastic film cover working time, rainfall duration and daily evaporation. When the operating time is constant, the plastic film cover of precipitation is decided by the rainfall duration and daily evaporation. According to the crystal pool of plastic film cover, two projects for decisionmaking of plastic film cover to precipitation are worked out.