Disclosed is a parallel-connected uninterruptible power supply system connected with a power source and a load. The uninterruptible power supply system includes a power distribution circuit having a first connector, a plurality of uninterruptible power supplies having a plurality of second connectors, and a plurality of transmission lines having a plurality of third connectors. The first connectors, the second connectors and the third connectors each includes a first conducting structure and a second conducting structure. When the second conducting structure of the third connector is detached from the second conducting structure of the first connector or the second conducting structure of the second connector, the second conducting structures are detached from each other first so that the pull-down circuit of the corresponding uninterruptible power supply generates a control signal to a controller to stop the operation of the uninterruptible power supply. Next, the third connector is detached from the first connector or the second connector so as to remove the malfunctioned uninterruptible power supply.