The invention discloses an on-line monitoring device of a passive optical network. The on-line monitoring device comprises an optical time domain reflectometry (OTDR) tester (3), a first optical switch (4), a first multiplexer (5), a second optical switch (6), a first de-multiplexer (8), a second de-multiplexer (7), a controller (9), a third optical switch (10), a third de-multiplexer (11), a forth optical switch (12), an optical splitter (13) and a plurality of second multiplexers (14), wherein a test signal and a digital signal which are generated by the OTDR tester (3) are subjected to coupling and decoupling of the above switch controller, the multiplexers and the de-multiplexers, the digital signal can be conveniently adjusted onto standby optical fiber, and the curve aliasing is prevented. By the on-line monitoring device, the problem of dark fiber test is solved, and on-line monitoring is really achieved.