Cavity ring down spectroscopy(CRDS) is an absorption spectroscopy technique with high sensitivity, which is a very important quantitative concentration measurement approach in combustion spectral diagnosis. The principle analysis of CRDS was presented to apply to absolute concentrations of intermediate species in flames, and then an experimental setup of pulsed CRDS was established based on this principle. The absolute OH concentration was measured by using absorption on the P1(2) rotational line in the A2Σ+-X2Π(0,0) band of OH on a plat flame at atmospheric pressure. Influence of several major factors on the measurement uncertainty was analyzed, such as laser line shape and linewidth, frequency stability of the laser, flame temperature, and cavity parameters. This uncertainty analysis provides several guidelines to improve the measurement accuracy of CRDS.