The invention relates to a multi-effect distillation exhaust system with preheaters for a seawater desalinating unit, which is mainly used as an exhaust system of a device taking steam, hot water and the like as heat sources and seawater as a raw material to prepare fresh water by a multi-effect distillation method. The system comprises a first effective evaporator (2), a second effective evaporator (9) until an n th effective evaporator, a liquid stop device (6), an evaporator seawater spraying device (19), a condenser (14) and a seawater feed pipe, wherein the seawater feed pipe along the seawater flowing direction is provided with a first seawater preheater (20), a second seawater preheater (21) until an n th seawater preheater; a foul gas discharge connecting pipe (26) is arranged between shell sides from the second seawater preheater (21) until the n th seawater preheater; the foul gas discharge connecting pipe (26) is arranged between the shell side of the n th seawater preheater and the shell side of the condenser (14); and a foul gas exhaust port (17) is arranged on the shell side of the condenser. The system improves water generating capacity in the same heat transfer area, and reduces the whole manufacturing cost of the device.