Due to the horizontal air duct and the improperly connected square diffuser, the airflow entering the throat of the diffuser causes large disturbance. Consequently, it makes the diffuser's air supply uneven from all sides，and eventually affects the distribution of indoor air. To study the effect of the throat structure on the uniformity of air flow, this paper, based on the computational fluid dynamic (CFD), combined practical engineering tests, calculated and analyzed the air distribution of improved diffuser under different wind speeds and proposed an improved scheme of diffuser, namely, increasing the height of the diffuser throat and setting guide vanes with adjustable angles in the throat. And an improved diffuser test platform was established to get the contrast test data through experiments and tests. The test data show that the improved guide vanes can more evenly balance the air supply of the diffuser on both sides of the perpendicular to the flow direction, so that the diffuser can easily ensure the uniformity of the wind on all sides, and the uniformity of air distribution increases significantly. As a result, the indoor comfort level is improved substantially.