Abstract: An ammonia decomposition reactor having a function of preheating ammonia gas, including a heat exchanger body and a reactor body enveloped externally by the heat exchanger body. A heat-exchange tube on the heat exchanger body is provided in a heat-exchange shell, one end is in communication with an ammonia gas heat-exchange inlet, and the other end is in communication with an ammonia gas heat-exchange outlet. A heat medium inlet and A heat medium outlet are individually connected to the heat-exchange shell. A catalyst tube is provided in a reaction shell. An ammonia gas heat-exchange outlet on the heat exchanger body is in communicated with an ammonia gas inlet on the reactor body, an ammonia gas inlet is communicated with an ammonia-gas-decomposition-gas outlet by a catalyst tube, and the ammonia-gas-decomposition-gas outlet is communicated with a heat medium inlet on the heat exchanger body.