The low-speed rectangular exit wind tunnel with sectioned contraction is widely used. The secondary flow vortices are found at contraction exit, which would lead to the non-uniform boundary layer and influence the aerodynamic experiment accuracy. In this paper, experimental and numerical approaches are adopted so as to clarify reasons for the formation of the secondary crossflow occurring at contraction exit and take measures to control it. The conclusions can be gotten as: the secondary crossflow is formed and developed in the second (rectangular-to-rectangular) contraction, and the first (circular-to-rectangular) contraction promote the secondary flow vortices to migrate to the middle of flow field to a certain extent; the formation of the secondary crossflow is related to the static pressure gradient in the contraction. Based on the mechanism analysis results, several methods aimed to control the secondary crossflow are proposed and verified, the results can be concluded as: in terms of the rectangular-to-rectangular contraction that contracts along one direction, it is difficult to effectively control the secondary crossflow just by optimizing contraction curves and contraction ratios, while adopting the boundary layer suction can significantly improve the boundary layer uniformity; if the rectangular-to-rectangular contraction contracts in two directions, such secondary crossflow can be well controlled.