Abstract: The bolt connecting the radar and the reliability test tool has broken in the reliability test of an airborne radar. In this paper the fracture reason of bolt is analyzed by macroscopic and microscopic morphological structure analysis, chemical composition analysis and metallographic structure analysis. The stress distribution of the fracture bolt is obtained by finite element simulation. The result shows that the chemical composition and crystal structure of the fracture bolt meet requirements of the standard. Excessive pre-tightening force of bolt installation, combined with orthogonal coupling effect of vibration, causes large stress concentration at the fillet knife groove of bolt root, which eventually leads to bolt fracture.