The principle of operation, characteristics and heat transfer process of horizontal tube falling film evaporators were introduced. Previous experimental correlations of the heat transfer coefficient on the outside of the evaporative tube and the heat transfer coefficient on the inside of the horizontal tube falling film evaporator was summarized. Analysis was conducted on the flow of the liquid film on the inside and outside of the evaporator tube under steady working conditions and the effects of evaporation temperature, heat transfer temperature difference, and tube diameter on the local heat transfer coefficient and the total heat transfer coefficient. The experimental correlation between the heat transfer film coefficient of the condensation side and the evaporative side of the horizontal smooth tube was compared. An empirical formula relating the condensation heat transfer film coefficient and the evaporation heat transfer film coefficient was derived which was suitable for the initial engineering design of horizontal tube falling film evaporators.