When the turbulence flows through a rectangular blunt body, due to the stretching and rotating motion of the vortex, the downstream fluctuating pressure has obvious unsteadiness and spatial three-dimensionality, which mainly depends on the ratio of the turbulence integral scale to structural characteristic size. In order to further study the mechanism of turbulence scale effect, the unsteady aerodynamic characteristics of fluctuating pressure rectangular downstream direction on stationary rectangular cylinder with side ratio B/D=4 were investigated by the three-dimensional spectrum tensor theory and synchronically surface pressure measurement. The results show that when the turbulence moves from the stagnation point of the rectangular section to the separation point, the spectral energy of the fluctuating pressure shifts from low frequency to high frequency, and this phenomenon becomes more obvious with the decrease of L_u/D (L_u is the longitudinal integral scale of turbulence, D is the windward height). In contrast, the three-dimensional effect of turbulence and the distortion effect decrease with the increase of L_u/D. During this process, the energy of the fluctuating pressure is significantly attenuated only at low frequencies due to the blocking effect, and there is no significant change at high frequencies. Finally, the introduction of three-dimensional pressure admittance reveals the unsteady effect mechanism of the fluctuating pressure on the windward side of the rectangular section under the action of turbulence. For the pressure at the stagnation point, the greater the L_u/D, the closer the pressure admittance is to the quasi-steady theory; for the non-stagnation point pressure, the farther it is from the stagnation point, the more obvious the blocking effect of turbulence will weaken the low-frequency energy and the weaker the three-dimensional effect of turbulence. The distortion effect is controlled only by L_u/D.