We design and fabricate a double-wavelength tunable laser with a single grating structure using a single-gain chip. The gain chip adopts an indium-rich cluster quantum constraint structure， which can generate ultra-wide flat-top gain. The flat-top gain is the basis for producing a dual-wavelength laser with the same intensity. A grating is inserted into the exterior of the gain chip so that its resonator is composed of internal and external cavities. The internal cavity consists of two natural cleavage planes of the gain chip for oscillating the output laser at a fixed wavelength （974 nm）. The tunable external cavity consists of a natural cleavage plane and a grating for the output laser at a tunable wavelength （969.1~977.9 nm）. The laser structures of the single-gain chip and single grating can produce synchronous dual-wavelength output， which avoids a complicated optical path design. The frequency difference between the two wavelengths is in the terahertz band. Thus， the laser can be used as a dual-wavelength laser source to generate terahertz radiation.