The effect of different connection modes of inductor polarity,namely,the start point of inductor winding on the stability of the current mode power supply is discussed. The main reason of the system instability is that the system operates in the critical state of current mode and voltage mode at low duty cycle. The inductor will cause the space magnetic field,which will cause the inducted current in the system feedback loop. Connecting the start point of the inductor winding to LX of Buck IC can strengthen the current feedback signal according to the inducted current,so the system can consistently operate in current mode without the oscillation. Conversely,connecting the start point to Vo will cause oscillation in Buck converter from 12 V to 1.1 V. The simulation waveforms and actual test results are obtained through the couple inductor model.