Abstract: Based on the solution of consolidation equations in which the soil layer thickness changes with time, this paper introduces continuous drainage boundaries, deduces and preliminarily proves the formula of pore pressure distribution and the formula of the average degree of consolidation, discusses the difference and connection between continuous drainage boundaries whose permeability changes with time and the undrained boundary in dredger fill engineering, the constraints of the Gibson formula for the average degree of consolidation. In view of the possible situation that negative pressure drainage system were located in the bottom boundary, a continuous drainage boundary was introduced to analyze the average degree of consolidation in different cases. Overall, the average degree of consolidation about soil layer thickness changing with time was studied in a certain depth. The conclusion has a certain theoretical significance and engineering reference value.