A plant lighting device, constructed at an indoor planting system, includes an illumination module and a power module. The illumination module includes an assembly seat, and an illumination unit including a plurality of light emitting elements that emit light toward at least one plant when powered up. The power module, mounted on the assembly seat, includes a power input line, a ring transformer module connected to the power input line and receiving an external power to perform power transformation, and a power output line connected to the ring transformer module and the illumination unit. The ring transformer module includes a ring core, a primary coil winding around the ring core and connected to the power input line, and a secondary coil winding around the ring core and forming magnetic coupling with the primary coil to output a non-harmonic alternating current power to the power output line.
