Abstract: In order to study the characteristics of unsteady aerodynamic forces (buffeting forces) on a bluff body, a rectangular cylinder is taken as an example. A pressure measurement technique was applied to study the span-wise buffeting force on the cylinder under turbulent flow with different integral length scales. The deficiencies of traditional 2-dimensioanl aerodynamic admittance were also studied by theoretical analysis, and a preliminary identification approach for 3-dimensional aerodynamic admittance based on statistical theory was proposed. The results show that the span-wise correlation of the buffeting force on rectangular section is larger than that of turbulence and is in proportion to integral length scale of wind fluctuations, which are similar to previous findings regarding streamlined bridge girders. The explicit difference between theoretical and tested results indicates that the traditional aerodynamic admittance can hardly reflect the span-wise distribution of unsteady aerodynamic forces. However, the 3-dimensional aerodynamic admittance with two-wave numbers can reveal the contributions of span-wise vortices with different dimensions to the buffeting force explicitly.