In this paper, a pulsed cavity-ring down spectroscopy (CRDS) is employed to measure the quantitative concentration of the OH radical in a plane flame burner with premixed methane/air. By analyzing the cavity ring-down absorption spectrum theory, we select the P1(2) absorption line spectrum of the electronic transition band OH A2Σ+-X2Π(0,0) and build a set of the pulsed CRDS experimental device with a laser wavelength of 308.6 nm. The device of the pulsed CRDS is composed of a pair of mirrors with a reflectance of 99.7%, the cavity length of the ring-down cavity is 270 cm, and the ring-down time of the empty cavity (without a flame in the optical cavity) is 2.33 μs. By analyzing the experimental parameters that affect the precise measurement of concentration, we use Planar Laser Induced Fluorescence (PLIF), Coherent Anti-Stokes Raman Scattering (CARS), and the pulsed CRDS to measure the effective absorption length of OH, high temperature of the flame, and cavity ring-down time. When the premixed methane (1.1 L·min-1) and air (15 L·min-1) are burned in a flat flame burner, and at the height of 6 mm from the burner surface, the precisely measured effective absorption length by PLIF is 7.1% higher than that of directly choosing the diameter of the burner surface as the absorption length, the measured precision of the temperature by CARS is increased by 45% than that measured by the thermocouple under room temperature, the measured precision of the optical cavity ring-down time with flame in the cavity and non-OH absorption wavelength is improved by 21.6% than that measured time of cavity ring-down without a flame in the cavity. By combining the above measurement techniques to measure all experimental parameters precisely, we obtain that the number density of OH molecules (3.59×1013 molecules·cm-3) can reach the maximum value when the height from the furnace burner is 6 mm, and the precision of OH concentration is 35.6% higher than that of the unmodified OH concentration. Under different equivalence ratios (Φ=0.7～1.1), with the increase of the height from the burner surface, the number of OH particles gradually decreases, and the curve fitting shows that the OH concentration decreases in an e-exponential decay. At the same combustion height, the concentration of OH increases with the increase of equivalent ratios. When the methane flow rate is kept constant, the OH concentration in the oxygen-rich combustion condition is higher than in the low-oxygen combustion condition. In the combustion field, the precise measurement method with the multi-spectral technology (CRDS-CARS-PLIF) can achieve the precise quantitative measurement of OH concentration and provide technical support for the quantitative measurement of the concentration of other combustion product molecules, which plays a crucial role in the study of combustion chemical reactions.