The design of device structure and the development of functional materials are the keys to achieve high efficiency organic light emitting diodes (OLEDs) over the past 30 years. Inverted structure device is considered to be a feasible solution for OLEDs to realize high efficiency and long lifetime. However, the electron injection barrier from the ITO cathode to organic layer is large, which limits the further development of the inverted device. In this paper, we introduce the related research work of n-type doping, dipole layer modification and tunneling injection, which bases on the different mechanisms of electron injection for inverted devices. Finally, three different strategies to improve the electron injection performance of inverted devices are summarized and prospected.