Abstract: HTR-10 is a type of pebble bed hightemperature reactor with spherical fuel element. Under high burn up irradiation, the stress due to high temperature irradiation will cause broken of fuel element. Therefore, to research the stress of the fuel element under high burn up irradiation is important. This paper introduces the structure of spherical fuel element, researches the temperature, stress and broken probability of fuel element. And also develops a program and judges the safety of fuel element through calculation.