Abstract: The traditional Iowa-type drilling shear tester (BST) has obvious shortcomings in terms of controlling shear rate, data acquisition, and soil adaptability. This study proposes an improved BST-I by optimizing the mechanical structure, developing a high-precision dynamic shear stress data acquisition system, and integrating an automatic displacement monitoring device, significantly enhancing testing accuracy and efficiency. Comparative tests show that the deviation in cohesion measurements decreased from 33.07% to 16.54%, and the deviation in internal friction angle decreased from 16.62% to 7.13%. The device has excellent depth adaptability, enabling continuous measurement of shear strength parameters of rock and soil at different burial depths, identification of potential sliding surfaces in landslides, and reduced time and labor costs. Normalized analysis using experimental data from Iowa State University in the United States revealed that the τ-norm of the improved instrument is significantly higher than that of traditional equipment, with greater differences between coarse-grained and fine-grained soils.To thoroughly investigate the response characteristics and applicability limits of the instrument under different drainage conditions, numerical simulations were conducted using Abaqus. Results indicate that under fully drained conditions, pore water pressure dissipates, leading to increased shear strength, whereas under undrained conditions, pore water pressure accumulates, reducing effective stress. Low-speed shearing is recommended under fully drained conditions to prevent excess pore water pressure, while rapid shearing is advised under undrained conditions to suppress shear swelling effects. At a shear rate of 3 mm/min, drainage conditions exert the minimal influence on shear strength. Therefore, this rate is recommended as the standard velocity for in-situ borehole shear tests.