Abstract: A 34CrNiMo6 steel spindle with excessive bending deformation had fractured during the hot straightening process. To investigate the fracture cause, macro and micro analysis, metallographic examination, energy spectrum analysis, chemical composition analysis and mechanical properties testing, and verification test were carried out. The results indicate that the fracture of the spindle is intergranular brittle fracture. When the spindle workpiece was straightened for a long time in the high-temperature tempering brittleness sensitive temperature range, high-temperature tempering brittleness occurred, resulting in weakened grain boundaries and increased material brittleness. When the stress induced by the external load exceeded the fracture strength, brittle fracture occurred. The accumulation of a large number of inclusions at the fracture source and other areas of the spindle promotes the occurrence of its fracture. It is recommended to avoid heating and repeatedly straightening within the temperature range of high-temperature tempering brittleness, and to improve the smelting and pouring technology to reduce the content of harmful elements and inclusions in the steel.