To investigate the wind load characteristics of rectangular sections with a small side ratio （D/B<1.0）， a series of three-dimensional models （with fixed cross-sectional area and side ratios ranging from 0.25 to 1.00） were adopted. First， the space-averaged large eddy simulation （LES） method was used to obtain the aerodynamic coefficients of four models with distinct side ratios， as well as the correlation coefficients of horizontal fluctuating wind pressure across different facades of the models. Subsequently， the influence of the side ratio on the product time-history peaks and phases of wind pressure at the side-surface measurement points was compared and analyzed. Finally， based on the dynamic mode decomposition （DMD） method， a reduced-order model capable of extracting dominant modes and reconstructing the wind pressure field was established， and the wind pressure fields of models with different side ratios were analyzed. The results indicate that a decrease in the side ratio affects the separation and reattachment of flow on the model’s side surfaces， altering the wind load action mechanism and significantly reducing the horizontal correlation of fluctuating wind pressure （with the 0.25 side ratio model exhibiting the most pronounced effect）. The transition point of wind pressure fluctuation characteristics is located approximately 0.6B from the side-surface reattachment region； the wind pressure time histories at the upstream and downstream of this transition point show opposite phases and negative correlation. The first four modes extracted via the DMD method can accurately reconstruct the flow field， thereby clarifying the random wind pressure field of small side ratio models. This study provides a reference for relevant flow control and structural design.