A systematic analysis is conducted to investigate the mechanical behavior of segment joints and to study performance differences between ultra-high performance concrete (UHPC) and reinforced concrete (RC) composite segments under complex stresses, induced by opening for metro connecting passages. Full-scale mechanical tests are carried out on longitudinal joints of UHPC-RC segments to evaluate their mechanical performance and observe flexural stiffness evolution under the combination of compression and bending. Additionally, a finite element parametric analysis is performed to compare the mechanical response and failure modes of the joints, and a simplified formula for checking the flexural stiffness of segment joints is proposed. The results are as follows: (1) At the ultimate state, the UHPC-RC joint exhibits a compression-controlled failure mode, with damage and cracking mainly concentrated in the compression zone of the RC segment, which is crushed as inclined cracks propagate. (2) The compression zone of the UHPC segment remains mostly structurally intact, with the appearance of microcracks. (3) Axial compressive load considerably enhances the flexural stiffness and ultimate bearing capacity of the joint; however, gradually diminishes with increasing axial force, displaying a prominent strengthening-saturation characteristic, along with a higher saturation threshold of the UHPC segment. (4) The flexural stiffness of the joint improves by ~15.5% when the concrete strength grade increases from C50 to UHC120. The simplified method for calculating the flexural stiffness of UHPC-RC segment joints is developed based on the experimental and numerical results, and can effectively predict the evolution of joint stiffness while providing a reliable basis for the design and performance evaluation of longitudinal joints in UHPC-RC composite segments.