Abstract: In order to promote the adjustment capability and measurement precision of electronic belt weigher, an automatic constant tension adjustment mechanism was designed on the basis of friction principle, wherein the tension roller and the adjusting roller were designed independently to eliminate the intervention between the two. Controlled by a PLC, the adjustment roller rotates towards the set direction; pushed by the axial component of the friction between the adjusting roller and the conveying belt, the conveying belt travels towards the set direction, as a result the tension of the conveying belt is automatically corrected. The designed mechanism was tested on a CK63E electronic belt weigher. The results showed that the adjusting roller could adjust the conveying belt bidirectionally and the tension of the conveying belt did not change while the adjustment mechanism was running, thereby the bidirectional and wide-range deviations of conveying belt become controllable and the measurement precision of electronic belt weigher is improved. This technique provides a support for promoting the control abilities in the process of tobacco primary processing.