The electromagnetic field of helix TWTs in the sheath model is analyzed. The dispersion and coupling impedance equations are derived and the one-dimensional space charge reduction factor is discussed in detail. The study shows that the difference of the space charge reduction factor between the sheath model and the perfect conducting tube is small at high frequency. The difference can be interpreted as the effect of the sheath helix. The higher frequency electromagnetic field has greater ability to affect the e-beam. Therefore, the space charge reduction factor equation based on the sheath helix model is inappropriate at high frequency.