Abstract: The wake field of a triangular prism is studied quantitatively in this paper by using particle image velocimetry (PIV) measurement, with different aspect ratios H(H=3.8, 1.9, 1.2, 0.9, and 0.7) at a Reynolds number Re=500. On the basis of steady and unsteady analyses, proper orthogonal decomposition (POD) and time-space cross correlation method are performed to analyse the influence of aspect ratios H on the coherent structure of the wake field, the steady-state transport zone, and vortex streamwise transport rate. The results show that, an increase in the width of the wake flow and the length of the wake vortex is generally related to a decrease in aspect ratio H, leading to vortex shedding slowing down and the unsteadiness of wake field weakening. A POD was performed to analysis the contribution of each mode on turbulent kinetic energy. It was found that the sum of high-order modal kinetic energy increased at low aspect ratio H with unstable wake field, decreased the steady-state transport zone and vortex streamwise transport rate. As a result, drag force coefficient increased with the increase in the width of the wake flow.