Si2N2O ceramics were prepared using amorphous Si3N4 as the raw material and Li2CO3 as the sintering additive through vacuum multi-stage sintering. The influence of the Li2CO3 addition (0%, 1%, 2%, 3%, and 5%, by mass) on the phase composition, the microstructure, the porosity, the mechanical properties, the dielectric constant and the tangent of the dielectric loss angle of the porous Si2N2O ceramics was investigated. The results reveal that a suitable addition of Li2CO3 can promote the generation of Si2N2O but excessive or inadequate Li2CO3 causes decomposition of Si2N2O ceramics. The prepared porous Si2N2O ceramics have good mechanical properties, good thermal shock resistance, and low dielectric properties, which have excellent potential for application in microwave sintering furnaces.