The surface lanthanum element plays a key role in emission mechanism of Lanthanum-contained cathodes. In order to gain insight into the relation between cathode emission properties and the cathode surface La/O ratio, lanthanum-rich film cathodes were prepared in situ by using a set of equipment, specially designed to cathode research, which connected with an Auger electron spectrometer (AES) and pulse laser deposition system. Electron emission properties and surface compositions (lanthanum and oxygen) of the cathodes are investigated experimentally. Experiment results indicate that film cathodes are lanthanum-rich film cathode, there has remained unchanged on film composition in different depth; 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. The excess active Lanthanum is crucial element to the improvement of emission properties.