Embodiments of the present disclosure provide a light-emitting diode (LED) dimming apparatus and an LED lighting system. The LED dimming apparatus includes: a voltage monitoring module configured to be connected between a positive terminal and a negative terminal of an LED power supply for an LED light or light string to sample an output voltage of the LED power supply; and a current controller with a voltage input terminal connected to a voltage output terminal of the voltage monitoring module, the current controller being configured to be connected in series with the LED light or light string to control a current of the LED light or light string according to the output voltage of the LED power supply. According to the embodiments of the present disclosure, the light intensity of the LED light or light string can be regulated by adjusting the output voltage of the LED power supply.