Piston jar is the important part of the quas-i fluid safety mechanism. Its mechanics property affect s the fuze’s funct ion great ly. Improper design of the piston jar w ill cause its g reater deformat ion upon firing the project ile, resulting in abnormal ex tract ion of the piston and leading to hard return of the rotor, as a result the fuse failure occurs. T he mechanics analysis is done on the piston jar w ith the ANSYS sof tw are, and the reasons of the fuze failure caused by the piston jar are analyzed in more detail based on calculat ion result s.