Abstract: To investigate the interfacial shear-bond performance between prefabricated ultra-high ductility concrete (UHDC) stay-in-place formwork and cast-in-place concrete, and the effective contribution of UHDC stay-in-place formwork to the shear capacity of RC beams, six RC beam specimens were designed. The study examined the influence of UHDC stay-in-place formwork on the shear behaviors of beams with or without stirrups. The results demonstrated that: (1) Using 20mm-thick UHDC stay-in-place formwork increased the cracking load by 18.7%~62.8% and the ultimate shear capacity by 44.1%~90.5%; (2) Embedding one layer CFRP mesh in the UHDC formwork provided only marginal improvement in shear capacity; (3) The enhancement effect of UHDC formwork was more pronounced in beams without stirrups, compared to the beams with stirrups. Based on the variable-angle truss model, considering potential shear failure modes in UHDC formwork beams (e.g., diagonal tension failure and shear-compression failure), a shear capacity calculation model was proposed. This model incorporates multiple factors, including longitudinal reinforcement ratio, stirrup ratio, shear-span ratio, concrete strength, thickness and strength of stay-in-place formwork.