A new kind of electron source, the photoemissive monoenergetic electron source has been invented for calibrating the beta-magnetic spectrometer. It produces electrons in the form of simulating a radioactive monoenergetic electron source and can be made in any shape and size according to the demands of experiments. In this paper, the principles and basic constructions of the photoemissive monoenergetic electron source are described, the results of calibrating our 2^1/2 π beta-magnetic spectrometer with a single strip arc shaped photoemissive monoenergetic electron source are listed, a new way for determining resolution function of experimental system in the research of neutrino rest mass has been posed and one of its actual applications is also given.