Honeycomb paperboards after dipping in acetylene carbon black were introduced to gypsum based microwave absorbing materials. Gypsum boards with single layer and double layer honeycomb structure were prepared and the microwave absorbing properties in the range of 218GHz were studied using N5234A PNA L microwave network analyzer. The results indicate that microwave absorbing properties of the specimens with single layer honeycomb structure are mainly affected by the size of each unit cell, including the side length and the height. The reflection loss of the gypsum board is below -10dB in the whole range of 818GHz when the side length and the height are 6mm and 8mm, respectively. For gypsum boards with double layer honeycomb structure, the microwave absorbing property is further improved due to the impedance gradient structure formed by different layers, including matching layer and absorbing layer. Different unit cell sizes are designed for two layers based on the impedance match principle. The optimal reflection loss obtained in the double layer gypsum boards is below -8dB in the frequency range of 218GHz and the lowest peak value could reach -203dB.