Conventional exhaust duct system design methods utilize the maximum load to choose equipment and ignore the actual running process. Therefore, in actual running process, exhaust duct system needs to meet the flow-rate requirements by adjusting the damper angles or the fan speeds. In addition conventional design methods fail to take into account future development and expansion of the exhaust duct system, which is critical to design concerns. In order to remove the disadvantages of conventional designs, feedback simulation method is proposed to simulate the actual running process. An acid gas exhaust system in a TFT-LCD factory is presented in this study. It demonstrates that the new duct simulation method contains a simple computation procedure and takes into account the balancing adjustment. The proposed design not only shortens the design timeframe and prevents human calculation errors, but also reduces the dependence on designer experience.