Abstract: To improve the durability of bridge piers in sea crossing bridges, a composite pier structure is proposed in which precast UHPC shells with high resistance to scouring and corrosion are installed on the outer surface of the pier in the seawater submerged region. However, the compressive behavior of this type of composite pier and the mechanical behavior of the interface between UHPC and normal concrete (NC) remain unclear. Therefore, using the piers of Luoyu Bridge as the prototype, one conventional reinforced concrete column and four composite short columns partially encased with precast UHPC shells were designed at a scale of 1∶7 and tested under axial compression. The height ratio of the precast UHPC shell to the short column, the thickness of the precast UHPC shell, and the presence or absence of an inner reinforcement cage were selected as the key parameters. The loading process, failure mechanism, ultimate load-carrying capacity, load-displacement response, and load-strain response of the specimens were analyzed. The results show that the use of an inner reinforcement cage, together with closely spaced stirrups at the interface, can effectively prevent local failure at the transverse interface between UHPC and NC. Increasing the height ratio of the precast UHPC shell to the short column significantly enhanced the confinement of the internal NC, thereby improving both the ultimate load-carrying capacity and ductility of the composite short columns. The ultimate load-carrying capacity of the specimen with a 300 mm high shell was 18% higher than that of the conventional RC column. When the height ratio of the precast UHPC shell was relatively low, increasing the shell thickness had only a limited effect on the load-carrying capacity, and slightly reduced the ductility. In contrast, the absence of an inner reinforcement cage led to a marked reduction in both the load-carrying capacity and ductility of the composite column, and the interface became the weak region. Finally, the load-bearing mechanism of the composite short columns under axial compression was investigated by analyzing the confinement effect of the precast UHPC shell. The findings provide a basis for the design and application of composite columns partially encased with precast UHPC shells in sea crossing bridge engineering.