A humidifying device for a continuous decarburization annealing furnace comprises a box, a gas inlet pipe and a gas outlet pipe. The gas inlet pipe and the gas outlet pipe are arranged on the box, a humidifying tank and a preheating tank are disposed in the box and provided with heating elements, a water inlet pipe which extends out of the box is arranged on the preheating tank, the lower portion of the preheating tank is communicated with the humidifying tank via a conduit, one end of the gas inlet pipe extends into the humidifying tank, the gas outlet pipe is communicated with the humidifying tank, a heater which is communicated with the humidifying tank via a conduit and positioned above the humidifying tank is further disposed inside the tank, and the lower end of the gas outlet pipe extends into the heater. The humidifying device is simple in operation, convenient to be controlled and high in heat efficiency and is widely applied to continuous decarburization annealing furnaces in various sizes.