Assembly stress from balance installation disturbs output value of component and affects measurement accuracy. In view of this, effect of float box bolt's fastening degree and thickness of float box on balance measurement accuracy were studied using finite element method without changing overall balance dimension. The results show that imbalanced assembly stress reduces measurement accuracy, floating box thickness also affects the measurement result. In test, wind tunnel balance should try to balance assembly stress, appropriate floating box thickness should be selected to improve accuracy of the measurement.