The invention provides a device and method for measuring the packaging pretightening force of a piezoelectric crystal actuator. The device comprises a positioning outer sleeve. The positioning outer sleeve comprises an upper cover plate and a base. A through hole allowing the piezoelectric actuator to be inserted is formed in the center of the upper cover plate, a threaded hole is formed in the center of the base, and a vertical stud is used for fixing a force sensor on the base. The method includes the steps that first, the piezoelectric actuator is placed on the force sensor through the through hole in the upper cover plate, the piezoelectric actuator and the upper cover plate are fixedly welded, then, upward fixing force is applied to the bottom end of the force sensor by screwing the stud, and the numerical value F1 of the sensor is recorded; the outer sleeve of the piezoelectric actuator is cut, and the upper end of the piezoelectric actuator is moved away; afterwards, downward force F2 is applied to a piezoelectric ceramic pile from the top of the piezoelectric actuator, the numerical value F2 is recorded until the numerical value displayed on the force sensor is F1, and the pretightening force of the piezoelectric actuator is F2-F1. According to the measuring method, mounting is convenient, measuring errors generated in mounting are eliminated, measurement accuracy is high, and the pretightening force of the piezoelectric actuator can be truly reflected.