Abstract: In the field of material dynamic mechanical properties research, it is significant to obtain reliable data of materials under complex stress states. To address the challenge of achieving a stable stress ratio during combined loading, this paper develops a novel device based on the electromagnetic Hopkinson bar (ESHB) platform. This device uniquely enables unilateral synchronous tension/compression-torsion combined dynamic loading. The paper details the device's configuration and loading principles. The core innovation of this device is the independent generation of trapezoidal tensile/compressive and torsional stress waves. Tensile/compressive waves are produced by a multi-circuit pulse shaper, while shear waves are generated using an electromagnetic clamp with torque storage. Crucially, a high-precision digital delay generator (DDG) ensures wave synchronization. With triggering accuracy within 0.1 μs, it controls the arrival time difference of these distinct waves at the specimen to within 5 μs. This overcomes the challenge posed by their different propagation velocities. Additionally, it describes the synchronization control methodology and the wave propagation analysis essential for timing calculations. To validate the apparatus, dynamic tension-torsion experiments are conducted on CoCrFeMnNi high-entropy alloys specimens. The results demonstrate the high reliability and effectiveness of the device. It successfully achieved a stable stress ratio of approximately 1.7 throughout the loading duration. Furthermore, the experiments conclusively showed a key finding. Trapezoidal wave loading significantly enhances stress ratio stability during combined dynamic loading. This improvement contrasts with the effect of traditional sinusoidal wave loading. This advancement offers a robust and controllable experimental method. It enables the study of materials' dynamic mechanical responses under complex stress states. These states involve high-strain rates and multiaxial loading. This capability is especially valuable for aerospace, impact engineering, and materials science applications. The successful implementation of constant stress-ratio loading opens avenues for more accurate characterization of material yield criteria and failure mechanisms under dynamic multiaxial conditions.