Since the wavenumber of distributed feedback (DFB) lasers related to its temperature and drive current, a method to determine the wavenumber is necessary. Measuring wavenumber with etalon is common, but only the relative wavenumber can be obtained. In this paper, a determination method for absolute wavenumber of DFB laser based on laser absorption spectrum profiles is proposed. The light of a DFB laser and that of a standard laser with known wavenumber pass through the same environment with stable temperature, pressure, and gas composition. By comparing the absorbance sequence from DFB laser and the absorption spectrum from the standard laser, rough absolute wavenumber is obtained. The rough absolute wavenumber and absorbance sequence from DFB laser are combined to acquire absorption spectrum. The absorption spectrum is compared with that of standard laser, and the wavenumbers of the points with a smaller error are selected to calculate the wavenumbers of points with larger errors by interpolation. It is repeated until the deviation of the two absorption spectra stabilize, and the absolute wavenumber of DFB laser is obtained. Simulations and experiments on the absorption lines of water molecules are used to verify the feasibility and effectiveness of the proposed method.