A multi-phase oscillator is provided. Said multi-phase oscillator includes a plurality of resonator stages series-connected in an ordered closed loop. Each stage is used for providing one or more oscillating voltages corresponding to an oscillating current. The oscillating current includes a natural current that is generated by the stage and one or more injected currents from a previous stage in the closed loop. The oscillating voltages provided by all the stages have substantially the same frequency; on the other hand, the oscillating voltages provided by each stage and the previous oscillating voltages provided by the previous stage have a corresponding phase difference. The oscillator further includes a coupler between each stage and the previous stage; the coupler is used for generating the injected currents according to the previous oscillating voltages. The coupler includes transconductance means for transforming one or more voltages corresponding to the previous oscillating voltages into one or more currents corresponding to the injected currents; the coupler further includes shifting means for shifting the phase of the injected currents according to the corresponding phase difference. The shifting means includes filtering means for filtering the previous oscillating voltages into one or more corresponding filtered oscillating voltages to be supplied to the transconductance means.