The invention relates to a device and a method for dehydration replacement of a reactor seed bed of a polyethylene device. The device comprises a seed bed treatment tank with a jacket temperature-control device, a nitrogen inlet pipe and its valve, a root valve, a seed bed feeding inlet pipe and its valve, a stirring device, an escape pipe and its valve, and a safety accessory. The method comprises that when a seed bed is transferred into the seed bed treatment tank by air or nitrogen, a temperature of the seed bed treatment tank is controlled in a range of 80-90 DEG C, refined nitrogen is fed into the seed bed treatment tank to improve pressure in the seed bed treatment tank to a certain pressure, the valve of the escape pipe is opened so that pressure replacement is realized, and the above processes are repeated more than 10 times until the content of impurities such water and oxygen of the seed bed is less than 10*10<-6>ppm. According to the method, fast removal of impurities of the seed bed is realized only by arrangement of the dehydration replacement device at a seed bed inlet of the original reactor without operation of a recycle gas compressor and a purge gas compressor. The method can prolong a device service life, saves raw materials and power, is safe and environmentally friendly and is simple and feasible.