In this paper, the unsteady flow field around the square bar is calculated with the IDDES (Improved delayed detached eddy simulation) model, according to the problem of aerodynamic noise generated by flow going by a square cylinder in the field of aviation. Due to the acoustic analogy integration solution of FW-H equation, the characterization methods of dipole and quadru-pole source intensity are established. Based on this, the distribution characteristics of the near field aerodynamic sound source around the square cylinder are obtained and the formation mechanism of the aerodynamic sound source is further studied by analyzing the unsteady flow field. The results show that the dipole sources are mainly distributed on the upper and lower surfaces of the square cylinder, which is related to the pressure fluctuation caused by the separated vortex formed at the front edge of the square cylinder flapping the upper and lower surfaces of the square cylinder; the di-rectivity of far-field noise generated by dipole source is also explained from the perspective of sound source distribution. The quadrupole sources are mainly distributed in the separation zone near the upper, lower and leeward surfaces of the square cylinder, which is related to the velocity fluctuation in the separation zone. The component along the flow direction of Lighthill stress tensor corresponds to the largest sound source intensity.