[en] An online measurement method is introduced to ensure the reflectivity of an arbitrary grating in a large-scale ultra-weak fiber Bragg grating (FBG) array. The measurement errors were investigated by theoretical analysis and experiment. In the proposed method, the reflected spectrum of one single FBG was addressed and obtained from the grating array by time domain interrogation, and the FBG reflectivity was calculated by the reference of the end-face reflection in anhydrous ethanol. A measurement error of less than 0.5 dB and a 20 dB dynamic range were obtained in a larger-scale array of 1000 FBGs with an average peak reflectivity of −43 dB, which demonstrated the feasibility of this online measurement method. (paper)