The wavelength-division-multiplexing passive optical network (WDM-PON) is considered as a promising technology for passive optical network (PON). With the increase of complexity and the expansion of scale, to ensure access network with the reliability and maintaining, the search for the physical-layer WDM-PON monitoring is therefore becoming crucial to the high quality and reliable system. The tunable optical time-domain reflectometry (OTDR) is an effective way, however, due to the complexity of WDM-PON fiber link, tunable OTDR also faces many challenges. By addressing the features of WDM-PON structure and major requirements of WDM-PON monitoring techniques, the research advances of tunable OTDR and the application of other technologies which are used for tunable OTDR are reviewed.