Abstract: The velocity fields behind the tandem double cylinders with Re=12 000 and spacing ratios L/D=1.167, 2.333, 3.500 and 4.667 were measured experimentally. The velocity distribution and turbulivity distribution at different sections behind the tandem double cylinders were analyzed. The flow field behind the tandem cylinders with small spacing ratio was analyzed by the stream function theory model, and the following conclusion was drawn: compared with the single cylinder, when the space between the tandem cylinders is small, the free shear layer of the rear cylinder is obviously shifted inward. As the spacing ratio increases, the free shear layer of the rear cylinder becomes more and more blurred due to the interference of the wake of the front cylinder to the rear cylinder. For the tandem cylinders with large spacing ratio, the disturbance to the rear flow field is strong, which leads to large turbulence intensity and maximum velocity attenuation in the rear flow field. Through the analysis of stream function theory model, it is found that under the condition of small spacing ratio, due to the mutual interference of two cylinders, the rear vortices of two cylinders are close to each other, and the angle of the rear vortices is reduced, which leads to the deviation of the free shear layer to the inside.