According to optical waveguide theory, the far-field profile of laser diode arrays is derived. Basing on the distribution, an off-axis external cavity is designed. By employing the external cavity, at the operating current of 17A, the BPP of output laser is reduced from 1100mm·mrad to 128mm·mrad and the beam quality has about 8.5-fold improvement. The output power is 75% of the radiated power in free-running state without external cavity.