We propose and experimentally demonstrate the monolithic dual-waveguide (DW) distributed feedback (DFB) laser with tunable wavelength spacing. The differences in the chirp sampled grating with various index modulation amplitudes are theoretically elaborated. The wavelength spacing properties of the DW laser at different Bragg spacings are compared and analyzed. To validate the numerical investigation, the DW laser consisting of three sections is fabricated and implemented, where the chirp sampled grating with two equivalent π phase shifts is located. The simulated relationship between the Bragg wavelength spacing and the mode spacing is consistent with the experimental results. Owing to the prominent contribution of the three-section structure and chirp sampled grating, the tuning range of the wavelength spacing is extended significantly, and the cavity of the DW laser becomes compact. The experimental results indicate that the proposed scheme achieves a tuning range from 59.50 to 116.25 GHz. The proposed scheme paves an extraordinary avenue for the integration of laser devices in the applications of optical sensing and THz communication.