A metal nanoparticles-composite thin film (Ag-BaO), doped by rare-earth elements, was grown by vacuum deposition. Compared with the normal Ag-BaO thin film, the photoemission current of the doped Ag-BaO thin film increases by about 40%. The transmission electron micrograph indicated that Ag nanoparticles become smaller when doped with rare-earth. With the smaller size of Ag nanoparticles, photoelectrons are easier to transgress from Ag nanoparticles via tunnel effect. Thus the photomission current increases.