This paper presents an adaptive harmonic injection mechanism for the single-switch three-phase discontinuous Conduction Mode (DCM) boost rectifier to provide a wide output voltage for power conditioning systems, such as pulse amplitude modulation (PAM) inverters and uninterruptible power supply (UPS) systems. Furthermore, the line currents of the single-switch three-phase DCM boost rectifier with constant frequency control contain relatively large fifth-order harmonic, which results in high total harmonic distortion (THD). The conventional harmonic injection mechanism is developed for the reduction of the fifth-order harmonic by injecting a constant sextuple harmonic signal to modulate the duty cycle at the fixed voltage-conversion ratio. However, the amplitude of the injected sextuple harmonic signal needs to be varied according to different input and output voltages. Therefore, an adaptive harmonic injection mechanism is proposed to vary the amplitude of the injected sextuple harmonic signal according to the sensed different input and output voltages for lower THD. Finally, a 1kW prototype circuit of the proposed adaptive harmonic injection mechanism for the single-switch three-phase DCM boost rectifier is designed and implemented to verify the input power factor correction (PFC) and line current harmonic for the fulfillment of IEC61000-3-2 Class A standard at wide output voltages.