Based on the internal relationship of thermodynamic and structure parameters of the low-temperature multi-effect evaporation (LT-MEE) desalination plant, calculation of the temperature difference distribution in a typical horizontal-tube falling film evaporator was carried out. Results show that the reduction of effective temperature difference caused by various thermodynamic losses occupies a large proportion of the apparent temperature difference in horizontal-tube falling film evaporator. The distribution of effective heat transfer temperature difference is extremely un-uniform in tube bundle. Under some conditions, the local effective temperature difference in the evaporator could be zero, and reverse heat transfer might even appear. Through the analysis of the thermodynamic losses in a desalination plant, the heat transfer critical point and the effective heat transfer critical point are proposed for better guiding the thermal design of multi-effect evaporation desalination plant.