Abstract: A construction machinery pin shaft cracked during service. The causes of cracking on construction machinery pin shaft were analyzed by means of macroscopic observation, chemical composition analysis and metallographic examination. The results show that the hardened martensite structure was formed during the pin shaft welding process, which producted a large internal stress. In addition, with the enrichment of impurities in the fusion zone at the interface between the welding material and the heat affected zone, a source of microcracks was formed. The microcracks expanded under the external force and eventually led to the brittle fracture of pin shaft.