The invention provides a gaseous phase dechlorinating agent. The gaseous phase dechlorinating agent is prepared from the following components in percentage by weight: 15 to 30 weight percent of calcium carbonate, 10 to 35 weight percent of magnesium carbonate, 7.2 to 10.8 weight percent of calcium oxide and 48 to 65 weight percent of aluminate cement. According to the dechlorinating agent provided by the invention, by using the characteristic that the calcium carbonate and the magnesium carbonate are used to generate a regular sandwich structure, chlorine is adsorbed more favorably, the dechlorinating activity is improved, and the breakthrough chlorine content of the dechlorinating agent at 300DEG C is as high as 55 percent; in addition, the strength can be improved, and the side pressure strength of the dechlorinating agent when clay is not used is as high as 110N/cm, so that the clay is prevented from being used and further the sintering of the dechlorinating agent is effectively prevented; meanwhile, by using high hygroscopicity of calcium oxide, the calcium oxide absorbs water and is converted into calcium hydroxide with higher alkalinity, so that the absorbing ability for hydrogen chloride is promoted, the dechlorinating precision is improved, the desorption of chlorine from the dechlorinating agent is hindered, the condition that water and metal chloride generated by chlorine absorption of the dechlorinating agent generate a coordination reaction is prevented, the damage to a dechlorinating agent structure and the blockage of an internal pore canal are avoided, and the chlorine capacity of the dechlorinating agent is effectively improved.