Abstract: There are more than ten plungers bushing cracking after grinding. The methods of macro-examination,chemical composition analysis,metallographic examination and hardness testing are employed to analyze cracked plunger bushing. The results showed that plunger bushing took place local overheat in heating process. Because of the local overheat,microstructure and austenite become coarse,reduce fracture resistance of material and increase cooling stress. These factors cause plunger busing to produce crack in quenching process. According to the analysis results,the improvement suggestions are put forward.