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A photovoltaic (PV) grid-connected power conditioning system with the function of uninterruptible power supply (UPS) is presented in the paper. It can not only realize photovoltaic generation, but also has full function of shunt active power filter (APF). In addition, it can act as UPS when instantly power interruption occurs. The instantaneous reactive power theory is employed in harmonics current detecting and signal synthesis algorithm of reference current. The proposed control strategy has two control modes (Grid connection mode and UPS mode) which can switch to each other automatically. A dual instantaneous loop control strategy is applied to UPS mode so as to obtain fast responses. The whole system is constructed and simulated in Matlab/Simulink environment. The power qualities under different conditions are given and compared. By analysis of the simulation results, it is shown that the proposed system is feasible and practical.