Oxide cathode has been widely used as electron emission source in vacuum electronic devices. Emission of an oxide cathode is dependent upon the ratio of the three kinds of ingredients and the microscopic shape of alkaline earth carbonates on the cathode surface. The synthesizing method plays a key role in dctermining the shape and ingredient of the carbonates. A gas-liquid synthetic method has been used to prepare the carbonate in this paper. The influences of temperature, concentration of constituents and whisk velocity were investigated. A nano-particle carbonate is synthesized by optimizing temperature and concentration.