The 1920 8.5 Haiyuan earthquake was the largest rupture in China in the twentieth century; however, the coseismic slip characteristics that provide insight into fault kinematics and future seismic hazards are unknown. In this study, we employed stochastic slip modeling to explore plausible slip distributions for this earthquake, incorporating different geodetic fault coupling models as prior constraints. Results demonstrate that fault coupling constraints have both positive and negative effects on stochastic models generating slip scenarios to fit the field‐measured geomorphic offset observations. Notably, a Gauss coupling model helps to generate surface slip with higher probabilities to fit geomorphic offsets, exhibiting a closer resemblance to the slip distribution of the Haiyuan earthquake. Integrating 41 slip scenarios of 6000 that reasonably fit the geomorphic offsets, we find that the Haiyuan earthquake likely involved multiple asperities and occurred as a cascading rupture event. The western and eastern fault segments may involve shallow slip deficits, implying potential distributed and/or off‐fault deformation during the earthquake, with implications for geomorphic offset–based interpretations of long‐term fault behavior. The modeling not only provides insight into the kinematics of the Haiyuan earthquake but also offers a plausible scheme for investigating historical earthquakes.