A polymer for an electron injection layer, an electron injection layer, an OLED device, and a display device are disclosed. The polymer includes a unit A and a unit B, in which the unit A is an aromatic group; and the unit B is a group represented by one of six possible formulae disclosed herein. The electron injection layer includes the polymer. The OLED device includes an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, the electron injection layer, and a cathode stacked in sequence.