The overall heat transfer process in a horizontal tube falling film evaporator is mainly influenced by the falling film evaporation outside horizontal tube due to the average heat transfer coefficient which is about 50% of that of the condensation inside tube. A series of experimental studies were conducted to investigate the heat transfer coefficients of the falling film evaporation outside the horizontal tube with parameters such as the spray density, the evaporation temperature, the salinity, and the tube spacing. Experiments were conducted by using Al-brass tubes with 19 mm outer diameter and 1600 mm length. The horizontal tubes are arranged vertically in the evaporator. The test tube is heated by an internal electric heater with uniform heat flux. Temperatures of the test tube surface and saturated vapor measured by thermocouples are used to calculate the heat transfer coefficients. The seawater with salinity of 1.5%, 3.0%, and 4.5% was used as experimental fluid. The spray density varied between 0.017 and 0.087 kg/(m s), and the evaporation temperature was controlled in the range of 50–70 °C. Results show that the average heat transfer coefficients of water under different salinities increase obviously with the spray density until a certain point. The average heat transfer coefficients of seawater decrease slightly with the evaporation temperature, decrease with the salinity, increase with the tube spacing, and are almost independent of the heat flux. In addition, the comparisons with 25.4 mm outer diameter tube and the circumferential distribution of local heat transfer coefficient are presented in this study.