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We develop a method to lock a high-finesse near-unstable Fabry−Perot (FP) cavity (F = 7330) to a frequency stable dye laser operating at 605.78 nm using the Pound−Drever−Hall technique. The experimental results show the feasibility of locking this cavity to different transverse modes. This method links the external FP cavity to the dye laser cavity, and a 379 kHz final linewidth of the FP cavity is achieved. Such a near-unstable cavity is potentially useful for cavity-enhanced spontaneous parametric down-conversion to generate narrow-band single photon or photon pairs in different transverse modes.