To promote the performance of NBTKBT leadfree piezoelectric ceramics and develop new leadfree piezoelectric stack actuators, the polarizing process of 0.8NBT0.2KBT piezoelectric ceramics has been studied by the mono factor analysis of variance and the orthogonal test. The results showed that the piezoelectric constant d33 reached to 108 pC/N with the poling field of 6 kV/mm and temperature of 70 ℃ for the poling time of 50 mins and holding time of 21 mins. A leadfree piezoelectric stack actuator with a new type of external displacement magnifying mechanism was fabricated on the basis of triangle amplification principle and flexure hinge structure. The results showed that the displacement of the directively driving stack actuator has been magnified 3.7 times and the remarkable displacement of 25 μm was achieved under a driving voltage of 150 V comparing with the stack actuator with external displacement magnifying mechanism. The leadfree piezoelectric ceramic stack actuator has the features of large displacement, good stability and lowcost, and will start a new scope of application to the microprecision positioning system.