The invention discloses a combined chord measurement method for longitudinal irregularity of a steel rail. Under the premise of no increase in the system cost, through comparison and selection of a particular chord length of 1m and combination of a cord length dividing ratio of 3:97, an optimal FIR inverse filter design method and flexible combination (three one-dimensional sensors simultaneously use a three-point deflected chord, and only 1# and 3# sensors use a two-point chord) of three sensors in the same group are combined, the short wavelength detection and the long wavelength detection are integrated, and actual irregular waveforms of the rail in a full recovery band can be accurately restored.