The invention provides a device and a method for processing slag by seawater and application of processed slag. The device consists of a slag granulating system, a seawater circulating system, a slag-water separating system and a steam collecting system, and the device also comprises a trapping water purifying system, wherein the steam collecting system obtains fresh water by steam generated in the process of condensation slag granulation, and utilizes the fresh water to wash the solid slag after granulation. The device can also be used for desalinating seawater, and the fresh water is obtained by condensing the steam generated in the slag granulating system. The device has the advantages of avoiding the use of external fresh water, reducing the salt content in the slag under a condition of not increasing the fresh water, and using excessive fresh water for other purposes; and simultaneously, the device does not discharge sewage or heated sewage to the sea, thereby seldom executing influences on the sea.