The embodiment of the invention provides an OLED light emitting device, a preparation method, a display substrate and a display device, and relates to the technical field of display. According to the technical purposes, the light extraction efficiency of the OLED light emitting device can be improved, and the visual angle can be increased. The OLED light emitting device comprises a reflecting electrode, an organic light-emitting layer and a semitransparent electrode, wherein the reflecting electrode, the organic light-emitting layer and the semitransparent electrode are arranged in sequence; the OLED light emitting device further comprises a light extraction layer which is located on the side, away from the organic light-emitting layer, of the semitransparent electrode and makes contact with the semitransparent electrode, the light extraction layer is of a single-layer structure in which the refractive index is decreased progressively in the light-emitting direction, and the light-emitting direction is the direction, away from the organic light-emitting layer, of the light extraction layer, wherein the refractive index of the side, making contact with the semitransparent electrode, of the light extraction layer is larger than the refractive index of the semitransparent electrode.