The development of a new-type photocathode for laser-driven high-brightness e-beam injector was reported.By the use of Cs+ ion implantation,a Mg-Cs alloy photocathode on Mg substrate was made.Some parameters of the ion implantation,the sputtering process,the photoemission and its mechanism were studied.Compared with the pure Mg photocathode,an order of magnitude improvement in the quantum efficiency from this photocathode has been obtained,meanwhile,we have also obtained a single-photon electron emission under the driving of 532nm green laser,the latter is a new development for the first time so far as we know in the world.Our experimental results provided a new way for the technology of high current,ultra short pulse e-beam.