In this paper, the analyses of the loss characteristics for a G-band improved folded waveguide slow wave structure are presented using two theoretical formulas of calculating the equivalent conductivity. The influence of four process errors on the slow wave structure are discussed, including the depth a, sidewall profile, beam tunnel alignment, and butt misalignment. The sample of the G-band improved folded waveguide slow wave structure is process and cold test, and then the transmission and loss characteristics are measured. Two theoretical formulas for calculating the equivalent conductivity are analyzed by the loss characteristics and the attenuation factor, the result shows the Hammerstad model is more suitable for calculating the equivalent conductivity at G-band.