In order to solve the problem of small wavelength tuning range of external cavity tunable diode lasers, the transmission characteristics of Gaussian beam in the resonator are analyzed. Results show that adjusting the output beam of gain chip to a small divergence angle to achieve collimation is not conductive to expand the wavelength tuning range. For a Littman-Metcalf tunable diode laser covering 1 480~1 640 nm, a collimation scheme is proposed by adjusting the waist position to equal the length of the external cavity. The simulation results show that the tuning range of the tunable diode laser can be greatly improved by using this collimation method compared with the collimation method with small divergence angle.