Signal of piezoelectric quartz force sensor has an offset in the influence of thermal. The thermal deformation of pretightening structure is analyzed under the hypothesis of uniform temperature, indicating that the thermal stress is the main reason of the offset. The change of thermal stress is analyzed by FEA software. The result shows that thermal stress of the piezoelectric quartz changes -3 315 kPa on the pretightening exerted direction. Finally the preload structure is improved and a contrast experiment is designed. During 240 s process of heating, the output of improved dynamometer on the pretightening exerted direction changes less than 4 mV, which indicates that the improved dynamometer can restrain the offset in the influence of thermal and improve the accuracy of measurement.