Abstract: To reveal the damage mechanism and limit state of UHPC strengthened piers under eccentric compression with secondary stress considered, the UHPC strengthened RC piers were designed and fabricated, and the corresponding experimental study and theoretical analysis were conducted and compared with un-strengthened RC piers. The small eccentric load tests of UHPC strengthened RC piers with section enlargement method considering secondary stress were carried out, and the failure mode and ultimate load capacity of the strengthened piers with different initial stress were studied. Finite element models of strengthened columns were established, and the influence of UHPC strength, UHPC thickness and longitudinal reinforcement ratio on the ultimate load capacity was discussed by employing the finite element model. Based on the mechanical characteristics and equilibrium relations in limit state, the formula for calculating the ultimate load capacity of RC piers strengthened with UHPC was derived. The test results show that the load capacity of UHPC strengthened RC piers with reinforcement mesh is 183% larger than that of the un-strengthened pier under small eccentric load. The ultimate load capacity of the strengthened piers increases with the UHPC strength. The UHPC thickness has an obvious effect on the load capacity of the strengthened specimens, while the longitudinal reinforcement ratio has small effect on the load capacity. The increase of initial load will reduce the concrete utilization of the original piers, as well as the load capacity of the strengthened piers. The calculated load capacity of the proposed formula considering the secondary force has an error of less than 3% compared with the measured value, and less than 8% compared with the finite element calculation value, showing a relatively high calculation accuracy. The research results can provide a theoretical basis for the calculation of load capacity of RC piers strengthened with UHPC under eccentric compression.