The invention discloses an online optical cable monitoring device, comprising an independent light source, a first optical distribution frame, multiple optical cables to be measured, a second optical distribution frame and multiple wavelength division multiplexing modules, which are arranged in sequence, wherein the wavelength division multiplexing modules are connected with the second optical distribution frame, each wavelength division multiplexing module is respectively connected with an optical power module and an optical switch module, the optical power module is connected with an operation control terminal, the optical switch module is connected with an optical time domain reflector, and the operation control terminal is connected with the optical time domain reflector. The invention also discloses an online optical cable monitoring method for improving the alarm accuracy rate. In the operation control terminal, the following judgments are made in sequence: whether loss data detected by the optical time domain reflector exceeds an alarm threshold or not, whether the alarm position is in a junction or not, whether the loss of the junction is in the acceptable loss range or not, and whether the continuous alarm times of the same optical fiber core of the same optical fiber exceed the continuous alarm times threshold or not. By the adoption of thedevice and the method, the alarm accuracy rate of online optical cable monitoring is effectively improved, and the situations of no alarm, short or instantaneous false alarm caused by artificial operation are reduced.