For a large range of ratio and small force piezoelectric dynamometer, the influence of the structure size on the test accuracy is analyzed. The result shows that the deformation caused by the insufficient stiffness of the dynamometer itself leads to the increase of interinterference between dynamometers and the decrease of test accuracy by combining the theory and finite element analysis. The upper cover thickness, the central aperture and the sensor span parameter are the key factors that cause the stiffness of the dynamometer to be insufficient and affect the accuracy of the dynamometer test. By optimizing the size parameters of a small force dynamometer, all the natural frequencies are above 4 kHz, and the dynamic performance is excellent. Under the action of nominal force, the maximum interinterference is 3.33% and the test accuracy is improved by 33.4%.