In order to study the flexural behavior of epoxy-joint segmental ultra-high performance concrete (UHPC) beams with external tendons, four-point bending experiments were completed for three UHPC beams with external tendons, including one monolithic beam and two segmental beams, and the main results such as the failure modes, load-deflection relationship and joint opening of the specimens were obtained. The epoxy-joint segmental UHPC beams mainly failed due to the damage of the epoxy as the joint was opened, which is different from the epoxy-joint segmental NC beams where the failure usually occurs in the NC near the joint. Considering the sectional pre-compression stress and the flexural tensile strength of epoxy, the calculation formula of the joint opening moment was proposed, and the difference between the calculation result and the experimental result was less than 1%, which was in good agreement. In order to accurately calculate the flexural capacity of segmental UHPC beams with external tendons, the Rod-Spring Hinge model that simplified the segments as rods and assumed the joints as spring hinges was proposed. Based on this model, the analytical method was used to derive the calculation formulae of stress variation and the secondary effect of external tendons. The difference between the theoretical and experimental values of tendon stress was less than 2.3%, which was in good agreement. Considering the uncertainty of segmental beams’ joint opening, a concept of the flexural capacity domain was proposed, i.e. the envelope range of structural flexural capacity under different joint opening conditions. The difference between the lower limit of the flexural capacity domain and the experimental values was less than 6%, so the lower limit is suggested as the reference value of the structural flexural capacity in a safe way. The above method can enrich the theory of calculating joint opening moment and structural flexural capacity for the epoxy-joint segmental UHPC beams with external tendons.