The rotary pipe evaporator includes a container which driven by external force makes rotary movement, and in the container there is a vertical heat-transmission pipe, and in the pipe a hard stirring rod capable of moving freely is installed the bottom end of said rod is supported on a horizontal plate. The rotary movement of said container can make the stirring rod roll along internal surface of the pipe so as to distribute liquid in the form of thin-film, thereby preventing scaling. The external surface of the pipe is equipped with a group of fins, and those fins can be used for centralizing and guiding condensed liquid so as to improve heat-transmission of condensation side. Those fins are preferred to be arranged longitudinally, and its stirring rod has the hydrophobic property. The evaporator can be used as desalination equipment of sea water. Besides, the evaporator is preferred to include a degasification device so as to prevent formation of "air space" on the external surface...