The quiet zone of terahertz time-domain spectroscopy experiment system was measured, which was very important for targets measurements. Experimental data had been processed with software packages of MATLAB and Origin respectively and corresponding fitting curves were gotten as a result. Relative to the travelling direction of electromagnetic wave, different fitting formulas including Gaussian were found describing the electric field intensity distribution very well in the cross section of the quiet zone transversely and vertically. However, there was fluctuation in quiet zone longitudinally. Additionally, the linear relationship between peak scattered electric field and metal sphere diameter was studied with this experiments.