Abstract: In order to reduce the variation of the water content of processing gas and thereby to reduce its influences on the moisture content and filling value of output tobacco from HDT pneumatic tobacco dryer, an automatic control system was designed. The optimal water content of the processing gas was determined via analyzing its relationship with the filling value of the tobacco to be dried. The water content was calculated according to the oxygen content and other parameters of the processing gas, and the actual water content was controlled within a range of set value±2% through adjusting the opening of an exhaust valve. The system was tested on the expanded cut stems for cigarette brand "Shuangxi (Lianxiang)" in Guangzhou Cigarette Factory. The results showed that the standard deviations of water content and temperature of the processing gas decreased by 51.37% and 27.66%, those of moisture content and filling value of output cut stems reduced by 13.33% and 36.96%, respectively; the filling value of cut stems increased from 6.69 to 6.83 cm³/g. This technology provides a support for promoting the quality of dried cut tobacco and cut stems.