The invention particularly relates to an energy saving composite oil tank for a hydraulic system. The technical scheme is that the oil tank comprises strip heat-conducting plates (1), radiating plates(3), an upper cover plate (4), a bottom plate (5) and a four tank wall plates (6), 12-100 strip heat-conducting plates (1) are vertically fixed on the four tank wall plates (6), each strip heat-conducting plate (1) is respectively connected with the radiating plates (3)via a core pulling hinge (2), and the bottom plate (5) of the oil tank and four edges of the upper cover plate (4) all extend out the four tank wall plates (6). The energy saving composite oil tank saves a heater and a cooling device, saves energy, and reduces the operation cost of the oil tank. Particularly, when the temperature of oil is higher than the normal usage temperature, then the radiating plates can be opened, when the temperature of oil is lower than the normal usage temperature, then the radiating plates can be closed, when variation of oil temperature is large, then the rotating angle of the core pulling hinge can be changed or the open and close amount of the radiating plates can be adjusted. As a result, the energy saving composite oil tank has the advantages of adjustable temperature, saving energy, being used beyond environment limit and having low operation cost.