The present invention provides an organic light emitting device and a fabricating method thereof. The organic light emitting device includes a thin film transistor substrate, a pixel defining layer, a reflective electrode layer, and a plurality of sub-pixels. The pixel defining layer is disposed on the thin film transistor substrate and includes a plurality of via holes; and the plurality of sub-pixels are disposed in the via holes correspondingly. A thin film stand layer is disposed on the thin film transistor substrate and corresponds to a portion of the via holes, causing a reduction in a depth of the portion of the via holes, and a depth of another portion of the via holes is greater than the depth of the portion of the via holes.