A gob-side entry retaining pasty filler on-line detection method comprises the steps that before each monitored space is ready to be filled, supporting rods are fixed to a base plate of a space to be filled through ground anchors, the upper ends of the supporting rods support a top plate of the space to be filled, measuring optical fibers are arranged between the supporting rods, an optical fiber connector is introduced out of the tail portion of the last supporting rod in each set of supporting rods and is connected in a mining optical cable through optical fibers so that optical fiber monitoring signals can be transmitted to a distributed optical fiber demodulator, demodulated signals are transmitted to a computer system, and pasty filler is monitored in real time on line and is analyzed and processed after the space is filled. The gob-side entry retaining pasty filler on-line detection method has the advantages that equipment installation is easy, the method is suitable for being universally popularized, no power supply is needed under a well, a site is safe, all-optical measurement and optical fiber transmission are adopted, influences caused by electromagnetic interference are effectively avoided, a monitoring result can be provided in time, and the method is favorable for directing on-site filling mining production work.