The invention relates to an evaporator of a low-temperature multiple-effect-distillation seawater desalting plant. The evaporator comprises a preheating tube bundle and an evaporating tube bundle, and is characterized in that the preheating tube bundle is arranged outside the evaporator; the evaporating tube bundle is arranged inside the evaporator; and the preheating tube bundle is connected with a residual heat source of the low-temperature multiple-effect-distillation seawater desalting plant. The tube bundles of the evaporator comprise two parts, namely the preheating tube bundle and the evaporating tube bundle. The preheating tube bundle of the conventional evaporator is moved out of the evaporator, material water is heated by using residual heat and other heat sources, so that the temperature of the material water entering the evaporator is further raised. In the invention, the internal space of the evaporator is effectively utilized, so that more evaporating tube bundles can be arranged, about 5 percent of an evaporating heated surface can be additionally formed, and the yield of a single-effect evaporator can be increased by about 5 percent.