We describe the wavelength calibration method of a narrowband laser diode in continuous-wave (CW) cavity ringdown spectroscopy (CRDS). The method uses known spectral lines as wavelength markers to calibrate and refine the wavelength-current relation of laser diodes, and their spectral positions are taken directly from the HITRAN 2004 database. We built a compact CW CRDS apparatus with a $1.517\mathrm{\mu m}$ ( $\sim 6594{\mathrm{cm}}^{-1}$ ) distributed feedback (DFB) laser diode as the light source and a $25\mathrm{cm}$ long glass ceramic as the cavity in which to demonstrate the method. A wavelength precision of $\sim 0.8\times {10}^{-3}{\mathrm{cm}}^{-1}$ was obtained by comparing the HITRAN 2004 database, which was approximately four times more precise than that of the conventional method.