Abstract: The emission stability, sustainability and reliability of cathodes are among key issues to be solved before the practical application of planar electron tube devices. In this article, the long-term continuous emission characteristics of a flat silicon cathode have been measured and analyzed in air and vacuum environment systematically. A stabilization process of about an hour exists for all samples in air while no such a process exists for samples in vacuum. The current sustainability between turn-on states has also been studied and the air ambient during turn-off states is found to be the main reason of emission un-sustainability. In addition, the microstructure features of the flat-type silicon cathode have also been revealed by TEM observation. A height drop of 5～6 nm on the silicon surface is observed along with obvious roughness in the emission area. The results in this work will help researchers improving the understanding of failure mechanisms in planar electron tube devices, and provide valuable guidelines for the design and operation of planar electron tube devices with flat-type silicon cathodes.