[Purposes] The paper aims to investigate the influence of the steady thunderstorm downburst wind field on the wind characteristics and wind loads of long-span bridges. [Methods] The steady thunderstorm downburst wind field was simulated with a thunderstorm downburst wind simulator. Taking a typical rectangular beam section as the research object, the study explored the influence of the steady thunderstorm downburst wind field on the wind pressure coefficient and wind load characteristics of the rectangular beam. [Findings] The thunderstorm downburst wind simulator used in this study can generate thunderstorm downburst wind field with high precision, meeting the conditions for simulating the steady thunderstorm downburst wind field. Within the scope of the radius r from 0.6Djet to 2.75Djet (Djet is nozzle diameter), the vertical wind profile of the horizontal wind speed of the thunderstorm downburst wind overall exhibits a shape of nose. The wind attack angles of the thunderstorm downburst wind at different heights are mostly negative, and within a certain height range, the absolute values of the wind attack angles increase with increasing height. Due to the influence of high air pressure below the nozzle, when the rectangular beam is located directly below the nozzle, the entire rectangular beam section exhibits positive pressure. As the radial distance increases, the thunderstorm downburst wind speeds gradually evolve from vertical direction to primarily horizontal direction, and the wind pressures on the surfaces of the rectangular beam gradually decrease. Within the range of r from 0 to 2.0Djet, the changes in the lift and drag forces of the rectangular beam are the most drastic, and the maximum values appear within this range. The trends of the lift and drag forces of the rectangular beam at different heights from the ground vary similarly with changes in radial position. [Conclusions] The research findings can provide references for the wind load distributions of bluff body structures such as bridges under the thunderstorm downburst wind.