<p>Biological motion exhibits bistable characteristics when presented in the depth dimension, and sound, as an important multisensory cue, can modulate this bistable perception. Previous studies often adopted nonbiological tones and had not fully controlled the inherent bias of visual stimuli. The underlying cognitive mechanism also requires further exploration using computational models. To address these research needs, the present study combined psychophysical methods with the hierarchical drift–diffusion model (HDDM) to investigate the effects of footstep sounds on bistable biological motion processing and its mechanism. A total of 24 naïve participants completed the experiment. Results showed that the proportion of “facing the viewer (FTV)” responses was significantly higher under looming and constant sound conditions relative to the receding sound condition, and reaction time (RT) in the no-sound condition was significantly slower than the other three sound conditions. Further HDDM analysis revealed that sound regulates the processing of bistable biological motion by shortening nondecision time (<i>t</i>) and modulating drift rate (<i>v</i>). The study demonstrates that footstep sounds accelerate the processing of bistable biological motion, and the directional information carried by sound drives visual perception to align with it. This effect is mediated by a two-stage mechanism that modulates nondecision processing (including early perceptual encoding) and strengthens evidence accumulation. This study provides empirical evidence for understanding the role of multisensory interaction in the perception of bistable biological motion. The data, materials and code are available in the Open Science Framework (OSF) repository (<a href="https://osf.io/3vm7p/">https://osf.io/3vm7p/</a>). None of the experiments was preregistered.</p>