<p>Drillstring vibrations are a critical concern in horizontal well drilling, often leading to premature component failure, well trajectory deviation, severe bit and stabilizer wear, and reduced rates of penetration. However, the underlying mechanisms governing these induced vibrations remain insufficiently understood. To address this issue, a novel experimental platform capable of simulating horizontal well geometries was developed to investigate the coupled dynamic behavior of drillstrings. Influencing factors, including rotational speed, feed rate, weight-on-bit (WOB), and formation lithology, were systematically examined. Dynamic responses were measured using force, torque, and triaxial acceleration sensors. To improve the accuracy of vibration characterization, the measured acceleration signals were transformed from sensor coordinates to a fixed geographic coordinate system based on Euler rotation theorem, thereby accounting for gravity and sensor installation eccentricity. Results demonstrate that increasing rotational speed excites nonlinear coupled vibrations and significantly amplifies both axial and lateral vibrations with distinct frequency-doubling characteristics. An increase in feed rate mainly intensifies lateral vibration, while simultaneously improving overall drilling stability by reducing fluctuations in WOB and torque. Increasing surface WOB suppresses axial bit bounce but aggravates lateral and torsional instability due to intensified drillstring buckling. Higher formation strength alters stick–slip behavior, resulting in a more stable rock-breaking process while slightly increasing axial vibration amplitudes. These findings provide new insight into the vibration mechanisms of drillstrings in constrained wellbores and offer a useful basis for drilling parameters optimization and vibration mitigation in horizontal drilling operations.</p>