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Chorus waves in the inner magnetospheres of the Earth and other magnetized planets typically occur in the form of discrete and repetitive quasi-monochromatic emissions with a frequency chirping, which was discovered more than 50 years ago. However, until now there is still no satisfactory explanations for repetitive emissions of chorus waves. In this Letter, chorus emissions excited by energetic electrons with a temperature anisotropy are studied by a one-dimensional PIC- simulation in a dipole magnetic field, and it is demonstrated that a continuous injection of energetic electrons caused by an azimuthal drift is essential for the repetitive emissions of chorus waves. Consistent with satellite observations, both discrete and continuous spectra can be reproduced. An intense injection of energetic electrons will lead to a decrease of the time separation between the chorus elements, and the chorus emissions evolve from a discrete to a continuous spectrum when the injection is sufficiently strong.