The present invention discloses one kind of under-driven mechanical finger device capable of preventing form degeneration. The device includes one first finger knuckle, one second finger knuckle hinged to the first one, one torsional spring installed to the second finger knuckle, one fixed base hinged to the second finger knuckle, one connecting rod, one buffer installed between the first finger knuckle and the connecting rod, one driving rod and one unidirectional rotation unit installed between the second finger knuckle and the fixed base. The under-driven mechanical finger device is used in flexible grapping manipulator, and has simple structure, reliable operation, small size, light weight and no need of additional control system overhead.