A new process of low temperature triple-effect distillation desalination is designed. Firstly, the appropriate effect number is determined. The total heat transfer coefficients and the heat loads of each evaporator are calculated. When the pressure differences between near effects are equal to △P, the freshwater outputs and the heat transfer coefficients of every effect would be increased as △P improved; if △P is too low, the seawater un-evaporated in the last evaporator would be difficult to flow into the next one. By analysis, the appropriate value of △P is 0.005 MPa. If the seawater flowing into the first effect is preheated by the heat source flow outpouring the unit, the total heat utilization rate and the freshwater output would be enhanced.