Waves excitation in the ionosphere by the modulated electron beams was observed in some active plasma experiments in space. In the active beam-plasma experiments electromagnetic wave can be excited by injecting modulated electron beam into the ionosphere, by various mechanisms according to different experimental conditions, The frequency of this sort of radioemission is determined by the beam modulation; it can lie in the whistler band or in the high frequency band （higher than electron plasma frequency）. This article gives a theoretical analysis as well as quantitative calculation of the radioemission of semi-restricted thin modulated electron beam injected from the spacecraft into the ionosphere plasma along the geomagnetic field. The result shows that when the modulated electron beams are injected along the magnetic field line, they can produce high-frequency electromagnetic radiations in the ionosphere plasma. This electromagnetic radiation is mainly observed in the plane that is perpendicular to the velocity direction of the injected electron beam. Estimations show that the radiated power is high enough to be detected.