This paper is the first part of series on across-wind forces acting on rectangular super-tall buildings. Seven rectangular super-tall building models with different side ratios were tested with simultaneous pressure measuring technique in a wind tunnel. Basic characteristics of the across-wind fluctuating force, including RMS force coefficients, power spectral densities, vertical correlation coefficients and coherence functions of the tall buildings are investigated. The effects of wind exposure condition and side ratio of the buildings’ across-section on the across-wind fluctuating wind forces are also studied. The results indicate that the side ratio has the most effect on the across-wind fluctuating wind forces; and the mechanisms of the across-wind fluctuating forces are possibly different for the buildings with different side ratios.