The invention relates to a method for directly separating rear earth elements in uranium dioxide or spent fuel. The method includes steps: (1) adding simulated spent fuel into functional ionic liquid,and well mixing to obtain solid mixed liquid; (2) stirring to dissolve the solid mixed liquid, and centrifugally separating to obtain supernatant liquid and lower undissolved uranium dioxide solid orundissolved spent fuel; (3) measuring the metal ion content of the supernatant by ICP-OES (inductively coupled plasma-optical emission spectrometer), and further working out the content of rare earthcompounds or other fission products dissolved in the ionic liquid; (4) directly subjecting the lower undissolved uranium dioxide solid or undissolved spent fuel to spent fuel element re-preparation.The method has advantages of simple process, simplicity and convenience in operation, high economy, environmental friendliness and the like, can be used for removal of rear earth compounds (neutron poison) and high-fission-yield fission products in spent fuel reprocessing and is suitable for industrial application.