Abstract: In order to study the surface emission mechanism of La2O3-Mo cathode,film cathodes were studied by using a set of equipment,specially designed for cathode research,with which film cathode can be prepared by pulse laser deposition.In order to gain insight into the relation between cathode emission properties and the cathode surface La/O ratio,electron emission properties and surface compositions(lanthanum and oxygen) of the cathodes were investigated experimentally.Experiment results indicate that the emission current densities descent gradually along with surface lanthanum film thickness attenuation;the cathode emission properties are determined intensively by the surface atomic La/O ratios.The higher the La/O ratio gets,the better the emission property reaches.Then the theory of La2O3-Mo cathode emission mechanism was discussed.It indicates that the theory of monoatomic layer can't explain perfectly emission mechanism of La2O3-Mo cathode;the excess active lanthanum is crucial element to the improvement of thermionic emission.