Two ultra-high performance concrete （UHPC） composite slabs with diamond-shaped joints （JF-L）and inverted T-shaped joints （JF-T） were designed and made to improve the wet joint of the UHPC-NC composite slab bridge in the coastal splash zone. An experimental study was carried out on these two specimens， and the interfacial stress was analyzed. The stress response of the entire progress from the initial crack to the failure of the composite slabs is obtained. Meanwhile， based on the experimental results and previous studies， the calculation formula for the interface cracking load of the UHPC-UHPC joint was developed. The results show that two specimens show bending-shear failure. The reinforcement ruptures at the reinforcement ratio changing section of the shear span due to the closer placement of reinforcement in the joint interface. The crack resistance and ultimate failure loads of the two specimens are nearly the same. The crack resistance is dominated by the interface behavior of the joint. Different configurations of wet joints have few impacts on the bending behavior of specimens. The nominal cracking stress of JF-L and JF-T is 4.71 MPa and 4.74 MPa， respectively， meeting the serviceability requirements of the coastal bridge. JF-L shows more cracks and lower cracking speed than JF-T. The UHPC-NC interface of JF-T is sound during the test， indicating great ductility and crack resistance of JF-T. The calculation formula for the cracking load of the UHPC wet joint was proposed and validated by experimental results in this and previous studies. In practical projects， the diamond-shaped UHPC joint is recommended as easy to prefabricate and construct.