Abstract: At present, psychrometric method is widely used to measure the relative humidity in the flue-cured tobacco chamber. The author discovered that when the temperature in the flue-cured tobacco house was higher than 40℃, the relative humidity value obtained by psychrometric method was different from that measured by the criterion instrument. And the difference is greater when the temperature is higher and the relative humidity is lower. Therefore, the author established the experimental data of psychrometric coefficient A related to temperature and wind velocity in temperature and humidity control room. By using approximation theory and least spuare algorithms, a new data-fusion formula of psychrometric coefficient A was presented. It is demonstrated that the error of the relative humidity value calculated from A value obtained by employing this formula is less than 1.5%R.H, in the range of wind velocity within 0.2～4.0 m/s, temperature within 40～70℃ and the relative humidity higher than 40%R.H.