A system for replacing a ceramic roll on line comprises a cooling device and a roll replacement device, wherein the cooling device is suitable for being nested on a ceramic roll bonding part required to pass through the inside of a box body of a slag box when ceramic roll is replaced; the cooling device comprises a main cooling body surrounding the outside of the bonding part and a gas feeding device for providing cooling gas, and a gap which is 0.1-0.5 mm wide is formed between the main cooling body and the bonding part; a cooling channel communicated with the gap is formed in the main cooling body, the gas feeding device inputs the cooling gas to the gap through the cooling channel, and the gas feeding speed and pressure of the cooling channel providing gas for the gap are higher than the exhaust speed of the gap and the pressure of the slag box respectively; the roll replacement device is used for pulling the ceramic roll out of the slag box. The system solves the technical problem that a ceramic roll in the prior art is not suitable for online replacement, has a better ceramic roll cooling effect and is more suitable for online replacement of the ceramic roll.