The invention provides an on-line spray gun replacement or overhaul device for a urea pyrolysis reactor. The device comprises a pyrolysis reactor body, a stiffening ring, a connecting pipe, a high-temperature-resistant gate valve, a compression air pipeline, a positioning flange, a non-metallic gasket, a lap joint flange, a positioning bolt, a sealing gasket, a sealing pipe clamp, a urea solutionspray gun and the like. The device system achieves on-line replacement or overhaul of the urea solution spray gun under the combined action of the lap joint flange, the high-temperature-resistant gatevalve and the compression air pipeline; when the spray gun needs to be overhauled or replaced, the compression air pipeline is opened first, the sealing pipe clamp and the lap joint flange can be loosened immediately, the spray gun is pulled out to the positioning flange, the high-temperature-resistant gate valve is switched off, finally, the flange connection is loosened, the spray gun is takenout of the pyrolysis reactor body, and therefore, an operator is prevented from being harmed by positive-pressure high-temperature exhaust gas sprayed out of the pyrolysis reactor body.