A cavity ring-down spectrometer was built up using a CW tunable Ti: sapphire laser. The experiments show that not only the spectral resolution is up to the 10-4 cm-1 level, but the detecting sensitivity also exceeds 10-10 cm-1. With the measurements of the absorption spectroscopy of C2H2 near 12 696.4 cm-1, the quantitative capability of the instrument was demonstrated. Through the spectroscopy of the samples mixed with trace C2H2 in N2 gas, the detection limit of C2H2 was determined to be 0.2 ppm.