Abstract: In order to establish a diffusion model suitable for backfill grouting in loess shield tunnels under low water content conditions, this study first calculated the permeability coefficient of the loess stratum, summarized common forms of seepage diffusion, and analyzed groutability criteria to comprehensively determine the screening diffusion form. A screening diffusion model was then developed, and calculation formulas for the filtrate diffusion distance and filter cake deposition thickness were derived. Finally, a parameter analysis was conducted to investigate the effects of the filter cake volume formed per unit filtrate, shield tail gap thickness, and formation permeability coefficient on filter cake deposition and filtrate diffusion. The results indicate that during backfill grouting in loess shield tunnels with low water content, the grout undergoes screening diffusion. Filter cake deposition and filtrate diffusion initiate and terminate simultaneously. The deposition time of the filter cake decreases with an increase in the filter cake volume formed per unit filtrate, and with a decrease in shield tail gap thickness and permeability coefficient. The maximum diffusion distance of the filtrate increases as the filter cake volume formed per unit filtrate decreases and the shield tail gap thickness increases, while variations in the formation permeability coefficient have little effect on the farthest diffusion distance of the filtrate. The findings of this study can provide a reference for controlling grouting stop time in engineering practice.