A tunable double-mode external cavity InAs/InP quantum dot laser based on a dual-Littrow configuration is demonstrated. A 115 nm single-mode tuning spectrum with obviously varied mode intensity has been obtained. A modified tuning method has been adopted to realize continuously tunable mode-spacing between two modes. It is found that single and double modes coexist in the tuning spectrum when one grating angle is fixed while the other grating is rotated; the biggest mode-spacing between double modes is about 75 nm. A continuously tunable range of mode-spacing between double wavelengths has reached up to 80 nm by inserting an attenuation slice in the external optical feedback paths to balance the intensity of the double modes; the separation of double modes is about 50 nm at best. The average output power of the double-mode laser is about a few milliwatts.