The present invention relates to a backlight dimmer circuit and a backlight dimming method. The backlight dimmer circuit includes a PLL dimming module, which detects a rising edge of a 3D synchronous signal to generate multiple channels of 3D mode dimming signal in phase with the 3D synchronous signal; a phase delay module, which generates time-divided outputs of the multiple channels of 3D mode dimming signal according to a delay set value; an external dimming module, which receives an external dimming signal to generate a 2D mode dimming signal; and a trigger, which receives the 2D mode dimming signal and the multiple channels of 3D mode dimming signal and determines whether to output the 2D mode dimming signal or the multiple channels of 3D mode dimming signal according to a 2D/3D conversion signal.