Abstract: Aiming at the structure principle of hydraulica driven variable-diameter threshing drum, this paper designed a hollow hydraulic cylinder that can realize reciprocating linear motion, which was mainly composed of outer cylinder, piston rod, inner cylinder, upper and lower end covers and various seals. Based on RecurDyn, the stroke and the load force of the hollow hydraulic cylinder was determined 28mm and 2.5 t, respectively. Through theoretical calculation, The outer diameters of the outer cylinder, piston rod and inner cylinder were determined 115mm, 80mm, 60mm, respectively. And the inner diameters of that were determined 140mm, 90mm, 80mm, respectively. Finally, through the performance test, it was verified that the diameter of the variable-diameter threshing drum can be adjusted steplessly through the hollow hydraulic cylinder drive adjustment mechanism.