Oxide cathodes have widely been used as electron emission source in vacuum electronic devices. Emission of an oxide cathode is dependent upon the ratio of the different ingredients of three kinds of alkaline earth carbonates on the cathodesurface and its microcopic shape. The synthesizing method plays a key role on the shape and ingredient of the carbonates. A new synthesis method，gas-liquid method，has been used to prepare the carbonates in this paper. The grains of the carbonates have filamental shape with sub-micrometer diameter. The testing results show that the cathode prepared with this sub-micrometer carbonate has large emission current density and long lifetime. The surface state and structure of the cathode are also analyzed with SEM in order to understand the mechanism of the enhanced emission current.