To investigate the flexural performance of precast assembled UHPC segmental thin-walled box girder bridges， this study is based on an externally prestressed UHPC segmental thin-walled box girder bridge with span length of 102 m. The cross-section of the actual bridge’s thin-walled box girder is equivalently modeled as an I-section. Subsequently， a four-point bending tests were conducted on three 3∶20 externally prestressed UHPC segmental beams to study the effect of different glued joint types on the flexural performance of the segmental beams， and the failure modes， joint opening width， deflection， and other related test results were obtained. The experimental results indicate that the main cracks in the segmental beams occur near the joints， with almost no cracks observed within the UHPC matrix itself. A calculation model for the stiffness of UHPC segmental box girders is proposed， which can accurately predict the stiffness of segmental box girders. This study provides valuable references for the further promotion and development of UHPC segmental thin-walled box girder bridges and enriches the calculation theories for externally prestressed glued joint UHPC segmental beams.