This paper presents the design for a highly sensitive balloon-borne measurement system based on cavity ringdown spectroscopy (CRDS) in order to analyze the vertical distribution of methane above the Tibetan Plateau. The proposed measurement system uses a digital signal processing (DSP) board to lock the cavity mode, acquire the cavity ringdown signal, scan the laser wavelength, and store data. Calculation of the decay rate, spectral curve fitting, and concentration calculations are all performed via DSP. First, the principle of CRDS and the spectrum calculation algorithm are introduced, including details on the improvement of calculation results by using a fixed Gaussian linewidth for fitting the spectral curve. Second, the cavity ringdown signal and spectral curve of the measurement system were analyzed, yielding a signal-to-noise ratio of approximately 62 dB for the cavity ringdown signal. The measurement system was calibrated in the laboratory by measuring standard gases; the maximum standard deviation of the measured values was 2.2×10-9, and the adjusted R-square between nominal values and the RMS of the measured values was 0.998 7. Finally, a field test was conducted in Lulang, Tibet. The instrument was carried by a tethered balloon, and methane concentrations at altitudes between 3 340 m and 6 000 m above sea level were successfully measured. Different trace gases can be measured by replacing the super mirrors in the optical cavity and using lasers with appropriate wavelengths, while the measurement system can be refined further to measure the abundances of certain isotopes.