The invention discloses an ammonia decomposition device, which comprises an electro connecting pressure gauge for detecting pressure inside a reaction cylinder, an electromagnetic valve which is communicated with the electro connecting pressure gauge, a pneumatic valve which is communicated with the electromagnetic valve, a nitrogen gas input pipeline which is connected with the pneumatic valve, a sealing plate and an expansion joint; when a furnace pipe has leakage, the electro connecting pressure gauge can generate a signal to control the electromagnetic valve which is connected therewith to be opened so as to control the pneumatic valve to open the nitrogen gas input pipeline, and the nitrogen is charged to the reaction cylinder to be protected, so the fire explosion accident can be avoided; by adopting the expansion joint, the expansion joint can absorb the displacement produced by the deformation of the pipeline caused by expansion and contraction and can protect the connection of the flange among the pipelines; through the sealing plate and the sealing connection of different parts, the closure and the explosion prevention of the entire device can be ensured, no irritant gasis escaped from the device so as to protect the environment; and when the ammonia decomposition furnace pipe is required to be examined or maintained, the device is convenient to disassemble.