Parameters of airflow in deep bed were obtained by using thermocouples and temperature-humidity sensors. It was concluded that the air flux directly affected the drying rate of grain. The drying rate ascended rapidly when the ratio of air flux to grain mass ratio increased in initial period. But when the ratio of air flux to grain mass increased to a certain degree and continued, the accretion of drying rate would decrease in evidence. It was also shown that a large air flux could greatly improve drying rate during the initial period of drying, but its effect was not obvious in the later stage. The movement rule of saturation relative humidity line in deep bed was studied on the basis of air flux and grain mass ratio. The relationship among optimal air flux and grain mass ratio, air temperature and initial moisture content was correlated to an equation. The parameters of the equation were given. The results provide foundations for auto-control of rough rice drying.