This paper discusses the principle of a large-aperture equal-thickness-fringe Fabry-Perot interferometer. Its merits include a relatively low requirement on optical material, high resistance to disturbance and vibration, and high accuracy of fringe location. A transient interferometer has been developed which has a large aperture and can operate in an environment with strong vibration. Interferograms of combustion flames and rocket exhaust plumes were obtained by using this interferometer.