Wavelength dependence of reflectivity at AR coating facets has been taken into account to study the wavelength tuning range of the external cavity semiconductor lasers. By applying the equivalent cavity method, an analytical expression for the threshold carrier density varied with wavelength has been derived. Analysis shows that the deviation between the wavelength of minimum reflectivity at the AR coated facets and that of the gain peak before AR coating is a key parameter to increase wavelength tuning range.