A plant illumination optical device includes two LED lamp bodies. Each of the LED lamp bodies includes a reflector and an LED lamp located at an inner side of the reflector, and an angle between the central light rays of the two LED lamp bodies is adjustable. A plant cultivation device contains the plant illumination optical device. The plant illumination optical device and the plant cultivation device can reflect the light beyond the planting area to the planting area and can supplement the illumination on both sides of the illumination centerline, thereby avoiding light waste, improving energy efficiency, improving the illumination uniformity in the planting area, and improving planting quality of the planting area.