In this paper, to reduce the cavity volume of a cavity ring-down spectrometer (CRDS), we propose designing a Z-folding cavity that has an external dimension of 26.4 cm×8.5 cm×4.5 cm and an expanded length of up to 73.8 cm. In the study, we perform a simulation based on the parallel-Gaussian-beam-transfer-equation to match the laser mode and cavity mode. By adjusting the focal length and position of two convex lenses, we realize mode matching and identify that only the TEM00 mode existes in the cavity. In the experiment, an acousto-optic-modulator is triggered by a digital-delay-generator to switch the laser beam. The feasibility of the Z-folding cavity is verified by the successfully acquiring ring-down signals. By exponentially fitting the experimental data, the ring-down time is determined to be 0.852 μs, which provides a maximum residual of 0.004 μW. The fitted ring-down time is consistent with the theoretical calculation result. The Z-folding cavity designed in this paper is compact and can be used in a commercial CRDS setup.