The invention discloses a large-diameter high-strength composite circular tube torsion test device and a test method thereof. The two ends of a disc of a circular tube upper fixture are connected through steel wire ropes, the pull force direction is changed through a fixed pulley, the two strands of steel wire ropes are combined together through a balance lever, and only one actuator cylinder is needed to easily and conveniently achieve the purpose of converting unidirectional pull force into torsional torque of a circular pipe test piece. Due to the fact that the torsional torque to which the circular tube test piece is subjected is equal to a product of the pull force and the diameter of the disc of the circular tube upper fixture, even though the pull force of the actuator cylinder is limited, the sufficient torsional torque can be supplied to a high-strength composite circular tube torsion test by increasing the diameter of the disc of the circular tube upper fixture to enable the circular tube test piece to be broken, the pull force of the left steel wire rope and the pull force of the right steel wire rope are equal, the pull force direction of the left steel wire rope and the pull force direction of the right steel wire rope are opposite, and the bending moment is exactly balanced, so that the circular tube test piece is only subjected to the effect of the pure torsional torque; accordingly, application of a pure shearing load is achieved, and the precision is higher than that of a general in-plane shearing test method.