A multi-carrier reflectometry device and method for on-line diagnosis of at least one transmission line. The device includes: a transmission part that includes successively: a module for parameterizing a test signal SF, such that: sF=[0,c1ejθ1,c2ejθ2, . . . ,CN/2-1ejθN/2-1,CN/2,CN/2-1ejθN/2-1,cN/2-2ejθN/2-2, . . . ,c1ejθ1]T, a module for synthesizing the test signal by inverse discrete Fourier transform, and a digital to analog converter, connected to the line; and a detection part that includes successively: an analog to digital converter connected to this transmission line, a discrete Fourier transform module, and an analysis module containing a deconvolution module, which also receives the test signal sF.