Disclosed are an offline uninterruptible power source and a control method therefor. The offline uninterruptible power source comprises: a safety switch (22) connected between an alternating-current input end (20) and an alternating-current output end (20′); a transformer (Tr2), wherein one end of a primary side thereof is connected to one terminal of the alternating-current output end (20′); a first switch (231), a second switch (232) and an output switch (24), which are successively connected between the safety switch (22) and the other terminal of the alternating-current output end (20′), wherein the first switch (231) is operable such that the safety switch (22) is connected to either of the other end or a tap of the primary side of the transformer (Tr2), and the second switch (232) is operable such that the output switch (24) is connected to either of the other end or the tap of the primary side of the transformer (Tr2); a third switch (29), which is operable such that the safety switch (22) is connected to either of the first switch (231) or the other terminal of the alternating-current output end (20′); and a bidirectional converter (25). The offline uninterruptible power source has high electric energy transmission efficiency.