The invention discloses a controlled synthesis method of a uranium oxide microcrystal/nanocrystal. According to the controlled synthesis method, a hydrothermal synthesis method is utilized to preparea pure substance of the uranium oxide microcrystal/nanocrystal, and the components of the uranium oxide microcrystal/nanocrystal comprise but are not limited to UO3.xH2O, U3O8, U4O9, UO2, and UO2.34.The microcrystal/nanocrystal prepared by means of the controlled synthesis method is high in yield which is larger than 98%, good in crystallinity which is larger than 95%, and good in controllability. The controlled synthesis method is simple, efficient and capable of regulating and controlling the components, the shape and the size simultaneously, and has great significance. Uranium oxide has wide application value, and the uranium oxide microcrystal/nanocrystal can be applied to an advanced nuclear fuel system in the future. Meanwhile, U3O8, UO3.xH2O, U4O9, UO2, UO2.34 and the like can be expanded and applied to the field of catalysis and the field of semiconductor devices.