The temperature difference function（TDF）reflecting the thermodynamic efficiency in the low-temperature multi-effect distillation（LT-MED）processes was developed，which provided a convenient tool to analyze the irreversible behavior and evaluate the thermodynamic efficiency of this system.Based on TDF，the effects of several key factors，including the total number of effects of evaporator，the terminal temperature difference of condenser，the heating steam temperature in the first effect and the ratio of thermal capacities，on the thermodynamic efficiency of LT-MED were investigated.The results showed that the exergy loss increased with increasing terminal temperature difference of condenser，increasing heating steam temperature in the first effect and increasing ratio of thermal capacities，but decreased with increasing the total number of effects of evaporator.