This paper proposes a new scheme of multistage injection locking controlled by a narrow-band CW dye laser to obtain spectral narrowing of a high-energy flashlamp-pumped dye laser (FLPL). The spectrum of FLPL with an output energy of 23 J was successfully narrowed to 0.7 GHz by this method. A narrow-band CW radiation was injected into the cavity of a small FLPL, then its locked output emission was injected into the cavity of the high-energy FLPL. With a CW injected power of 15 mW, about 70 percent of the output energy of the final laser was concentrated into the same spectral region as the injected CW radiation. The effective power gain obtained was as large as . Using coupled rate equations based on a simplified model, the injection-locking phenomena were analyzed. This model makes it possible to derive the locking efficiency as a function of the injected power in an explicit form, and gives an insight for experiments. Some performances of injection-locked operation were studied experimentally.