The invention discloses a reactor fuel element, comprising an upper-end plug, a core block, a jacket, a lower-end plug, an exhaust pipe and a buffer cup with an open upper end, wherein the core block is arranged inside the jacket; the upper-end plug and the lower-end plug are arranged at two ends of the jacket, respectively; the buffer cup is arranged inside the jacket and between the upper-end plug and the core block; a gap exists between the buffer cup and the jacket; the buffer cup does not contact the bottom of the upper-end plug; the exhaust pipe passes through the upper-end plug; the lower end of the exhaust pipe is arranged inside the buffer cup. The reactor fuel element has the advantages that the problem of increased internal pressure of an existing fuel element caused by a lot of fission gas is overcome, and a refrigerant can be prevented from flowing back.