Abstract: A modular composite device for measuring agronomic trait parameters of tobacco plants was designed to improve ease of operation and error reductions in manual measurement during current tobacco production. The device had mechanisms of measuring diameter, stalk-leaf angle and leaf length. The clamping plate structure for measuring diameter was determined to be double-arched by comparative analysis. The force analysis of the clamping mechanism during operation indicated that the minimum tensile force for the spring was 24.6 N. According to the national standards, the spring parameters were determined to be d₁=1.6 mm, d₂=8 mm, n=40.5, k₂=3.12 N/mm, and the shear modulus G was 79 GPa. The reasons for the errors in the stalk-leaf angle measuring devices were analyzed in connection with the physical characteristics of tobacco leaves, and the rod dimensions of the stalk-leaf angle measuring mechanism and the leaf length measuring mechanism were determined based on the agronomic trait parameters of mature tobacco leaves as follows: the clamping plate length was 40 mm, the adjustment range of the angle measuring mechanism was 30°-80°, the length of the movable rod was 450 mm, and the length of the connecting rod was 70 mm. The results of the field verification test of the measuring device showed that the device could accurately measure the stalk-leaf angle, leaf length and stalk diameter of tobacco plants with average errors of 0.48%, 0.88% and 0.13%, respectively. The overall measuring efficiency increased by 8.3%, meeting the design requirements. In summary, the device can be used to measure the agronomic trait parameters of tobacco plants.