Background <p>Biaxial tensile testing is key to studying the mechanical response and failure of composite materials under multiaxial stress states. However, many of the biaxial devices based on a uniaxial machine in research are limited to equiaxial loading, small deformations, and low loading rates, making them unsuitable for evaluating large deformations of soft materials in complex loading scenarios.</p> Objective <p>The objective of this study is to develop a connecting-rod variable-ratio biaxial device based on a uniaxial testing machine. The device was specifically designed to facilitate examination of the mechanical behavior of soft materials subjected to substantial deformations under different stretch ratios.</p> Methods <p>The principle of geometric similarity, derived from the connection structure, was introduced to achieve variable-ratio loading. Furthermore, a multistep staircase method was proposed to ensure homogeneous loading in the horizontal plane.</p> Results <p>Experimental evaluations of actuation synchronization, speed stability, and ratio effects indicate that the developed device provides stable and uniform motion and variable-ratio loading over a wide stroke range. Variable-ratio biaxial tensile tests were performed on soft material undergoing large deformations at 10⁻⁴ and 1/s, and the influence of strain rate effect on mechanical properties was analysed.</p> Conclusions <p>The improved biaxial tensile device presented in this study features a simple design and delivers stable and reliable test results, demonstrating suitability for the mechanical characterization of soft materials under complex stress conditions.</p>