With increase of the dynamic force test range and the improvement of test accuracy requirements, the key role of piezoelectric test technology becomes gradually prominent, which has been widely used in heavy-duty equipment and aerospace vector force measurement. In the process of assembling and applying preload of the piezoelectric three-way force measuring unit, problems such as the deflection of the upper plate and lower plate and the different axis incurred. Based on the load sharing principle of force, this paper quantifies the error and establishes a theoretical model of the self-preload error of the force measurement unit. A new type of assembly fixture is designed, and the angle deflection test experiment and static calibration experiment of the force measuring unit assembled with two types of fixtures are carried out. The experimental results show that the test accuracy of the force measurement unit assembled with the new fixture has been significantly improved, and the maximum value of interphase interference is reduced from 6. 06% to 2. 97%, which provides theoretical support for the improvement of the accuracy of the force measurement unit and the development of related structures.