In recent years, optical network is developing towards high rate and wide bandwidth. As the number of network equipments in optical network increases, the network power consumption has a sharp rising. In order to decrease the energy consumption and improve the energy efficiency, the concept of green IP over WDM network has been proposed and it has become a hotspot for research in the area of optical network. The existing research status for energy-saving approaches in optical network is reviewed. Specifically, starting with network equipment, the relationship between traffic load and energy consumption of network equipments is discussed and two power consumption models are established. Then a mathematical expression of minimized energy consumption for IP over WDM network is introduced based on the network structure. Subsequently, the methods to reduce the energy consumption at network level are divided into different categories, whose fundamental principles and energy saving effects are summarized, and several typical energy-saving approaches are compared as well. Finally, the limitations of existing approaches are analyzed and some suggestions are provided for the further research directions.