The pore structure of calcium oxide powder was modified by addition of small amounts of inorganic salts as impregnants to pulverized limestone, which was then calcined in a powder-particle fluidized bed (PPFB) at ambient pressure. The effects of type of fluidizing gas, temperature distribution along the vertical direction of the reactor and addition of inorganic impregnants on the physical properties and moisture absorptivity of the product powder were investigated. Dependence of specific surface area, pore volume and calcination conversion on the moisture absorptivity of the product powder under low dew point of 270-273 K was analyzed. Further calcination occurred in the freeboard region with higher temperature, giving a product powder with more porous structure. Nitrogen gas was superior to dry air for producing highly porous calcium oxide powder. Calcination with additive impregnation of 1% sodium carbonate and 1% calcium acetate expanded the size of pores, newly producing pores of larger than 10 nm. Disclosure tests under low dew point atmosphere revealed that the pore sizes of the product powder effective for moisture absorption lay in the range of 6-50 nm. Good correlation of moisture absorptivity was found in the initial stage with the specific surface area, and in the middle stage with the pore volume of the product powder.