To study the failure mechanism of the spark plug shell, the failure characteristics of the spark plug shell at different positions, such as inside, outside and section of the shell, were studied based on the torque loading experiment and the microscopic observation technology. The results show that the failure mode of the spark plug shell is plastic deformation, and the failure location occurs on the outermost ring of the shell thread; cracks on the inner and outer surfaces of the shell originate from the inner and outer surfaces of the shell rather than the interior of the material, and crack propagation on the inner and outer surfaces occurs separately; the final fracture of the shell is caused by the intersection of cracks on the inner and outer surfaces of the shell.