The invention discloses a total harmonic distortion adjusting method and circuit in an AC direct-connection LED driving IC. The method comprises the steps: converting a voltage output signal of each constant current module in the AC direct-connection LED driving IC into a current signal through a plurality of THD adjusting modules and external resistors RTD corresponding to the THD adjusting modules, and superposing the obtained current signals in the form of a current source, thereby achieving the changing of the voltage values of the LED current control end in different time periods togetherand achieving the changing of the current value flowing through the LED lamp string in different time periods, so as to enable the waveform of the current flowing through the LED lamp string changingalong with time to be closer to a sine signal and achieve the purpose of adjusting the total harmonic distortion of the AC direct-connection LED driving IC. Moreover, the adjusted value can be adjusted by changing the resistance values of a plurality of external RTD resistors, and can be selected according to different requirements.