The lifetime of a new type of reservoir oxide cathode and a conventional oxide cathode was experimentally studied. Concentrations of Ba in different coating depths of the cathodes were analysed by XPS during the lifetime testing process. The mechanism of the reservoir oxide cathode lifetime was discussed according to the analysis results, the cathode structures and the characteristics of the reservoir emission materials. The results show that the lifetime of the reservoir oxide cathode is about seven times that of the conventional oxide cathode. Ba concentration keeps steady in the depth of cathode coating, which plays an important role in the long lifetime of the reservoir oxide cathode.