Abstract: Owing to the superior mechanical properties of steel-HPEC(hollow-core partially encased composite) spliced beams and middle partially encased composite braces(MPEC braces), a multi-story X-braced HPEC frame was proposed. To evaluate the seismic performance of the composite frame, two composite frame specimens were constructed and subjected to horizontal low-cycle cyclic loading tests, and the finite element(FE) simulation was conducted through ABAQUS software. The computational outcomes indicate that the composite frame displays a good seismic performance and energy dissipation capacity. The failure mode of the specimen involved the instability of the braces, with no appreciable damage to the bare frame structure upon the failure of brace members. Filling concrete in the MPEC brace served to enhance both the stiffness and compressive performance of brace members while reducing the decrease in frame stiffness resulting from instability of the bare steel braces. The energy dissipation ability of the MPEC brace was greatly impacted by the H-shaped steel flange thickness. The analytical technique proposed and calculation formula could accurately predict the initial lateral stiffness of the composite frame, providing ideas and foundations for researching and designing other multi-story X-braced HPEC frames.