A new technique of using Optical Time Domain Reflectometry (OTDR) to detect the reflected power of FBG sensing array was researched systematically. The theoretical model between the reflected power detected by OTDR and the reflected wavelength of FBG was established by analyzing, and consequently the selection rule of FBG wavelength in OTDR detection system was proposed. Moreover, experiments about multi-combination of FBGs with high reflectivity were carried out. The cause of dummy peak and resonance peak in OTDR test curves were analyzed, and meanwhile the selection basis of FBG reflectivity was also given. Based on time division multiplexing theory, detecting low-reflectivity FBG by OTDR can increase the FBG multiplexing number greatly, which has wide prospect to be applied in FBG array detection.