The flow around a circular cylinder is a classic fluid mechanics problem. In order to reveal the relevant mechanism of the parallel double-splitter plate affecting the flow field around a circular cylinder, based on the commercial software, the SST-DES method is used to analyze the three-dimensional cylinder with a length-to-diameter ratio. Numerical simulation of the flow around a three-dimensional cylinder with a length-to-diameter ratio of 7r is carried out. First, the time-averaged drag coefficient, Strouhal number, and pulsating lift coefficient of the calculation results are analyzed and compared, and the accuracy of the numerical calculation result is verified. On this basis, a rigid parallel double-splitter plate on the incoming flow side is added to the cylinder. In the case of Reynolds number Re = 3900, the numerical simulation of the flow around a circular cylinder installed with a splitter plate is further carried out. Due to the influence of the splitter plate, the surface pressure, lift and drag coefficients, and wake field of the cylinder will change differently compared with the bare cylinder. Based on the results of the numerical simulation, the influence of the parallel double-splitter plate on the incoming flow side of different lengths on the flow around a three-dimensional cylinder is discussed and analyzed. The research results can provide a certain reference for understanding the mechanism of the flow around a three-dimensional cylinder and suppressing the vortex shedding of the cylinder structure.