Abstract: When the tool strength and sealing performance of a casing sliding sleeve were tested, the liquid outlet part suddenly broke. The causes of the sleeve fracture were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscope analysis, hardness test, impact performance test and metallographic examination and finite element analysis. The results show that the structure design of the casing sliding sleeve was unreasonable and there was obvious stress concentration at the outlet, resulting in a notch effect. The improper heat treatment process significantly reduced the fracture toughness of the material, which eventually led to the fracture of the outlet of the casing sliding sleeve.