An OTDR light reflection structure of optical sub-assembly for transceivers of PON system includes: a retainer member installed at an end section of a light guide unit of the optical sub-assembly for transceivers and positioned in the optical sub-assembly for transceivers, the retainer member having a first end and a second end; and a light reflection filter affixed to the first end of the retainer member. When an OTDR emits a specific wavelength of light, the light reflection filter directly reflects the light back into the light guide unit. Then the signal is transmitted back to the OTDR for the OTDR to identify the subscriber position of the feedback signal and judge whether the optical fiber line is in a normal state.