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In order to study the effect of cross wind on a moving object, a large-scale low-speed free-jet facility will be built with the uniform region of flow field about 180m wide, 1.1m high and1.5m long, in which flow speed can be changed continuously from 3m/s to 10m/s.Due to the large size in width, which is seldom seen in published bibliography, many problems appear such as general structure layout of the facility, calculation of aerodynamic contour, and so on;due to the low cost for its construction and high quality requirements for its flow field, it is difficult to completely use the structure layout of wind tunnel.On the basis of past experience, a general structure layout was put forward at first, then, to ensure the feasibility of the layout, a series of optimization tests were conducted on some test units to obtain experimental data of the flow field characters for the modification of the general layout, finally, a general structure layout of the facility which satisfy all the design requirements for the facility was determined according to the experimental data obtained in optimization tests.In this paper, the method and process of optimization tests on the general structure layout are introduced, and the different test units with their aerodynamic contours and related aerodynamic calculations, along with measurement instruments and measurement method are presented in details;the flow fields of the test units were carefully measured and the experimental results of optimization tests and the photographs of corresponding test units are also given in the paper.