The present invention relates to a dynamically adjustable LED plant light supplementing system and a dynamic dimming method. The system comprises an RGBW light supplementing lamp, a light supplementing lamp drive unit, an illumination intensity acquisition unit, a plant growth monitoring unit, a centralized control unit and a planting intelligence library. By utilizing the plant growth monitoringunit, the planting intelligence library and the illumination intensity acquisition unit, the illumination intensity, the light quality ratio, the light period and the illumination time distribution ofa plant lamp can be adjusted dynamically and automatically according to the plant type and the growth period, illumination parameters including a light quality combination, the light intensity of each single light quality, the running period of each single light quality and the alternating period of different light qualities can be set according to a light recipe, the growth stage of a plant is automatically judged without manual intervention, the light recipe is dynamically switched, any controllable illumination condition is provided in each growth stage, the plant growth of different plants is promoted, the plant quality is regulated and controlled, and efficient culture is realized.