Chorus waves are well known for their significant roles in the radiation belts of the Earth and other magnetized planets, including acceleration of electrons to relativistic energies, and precipitation of energetic electrons into the ionosphere to produce diffuse and pulsating aurora. They 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 report, chorus emissions excited by energetic electrons with a temperature anisotropy are studied by both a one-dimensional \(\delta f\) simulation and theoretical model in a dipole magnetic field. The two models have unanimously 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.