The present invention discloses a LED dimming method and a LED dimming system, wherein the LED dimming method comprises the following steps: A: calculating the equivalent current flowing through the LED in accordance with the duty cycle message of the input PWM signal; and B: regulating the current flowing through the LED to the equivalent current. Because the equivalent current flowing through the LED is calculated by the duty cycle message D of the input PWM signal while the duty cycle is not actually regulated, the present invention realizes the aim that the dimming uniformity is improved by linearly regulating the current flowing through the LED. Meanwhile, transformer noise will not be generated because current flows through the transformer in the overall dimming cycle.