Abstract: The consolidation theory is an important component of soil mechanics, and the coefficient of consolidation is generally elaborated as a key concept regarding the consolidation theory in textbooks. Based on the definition of the coefficient of consolidation, this study investigates its changing behavior during consolidation using the measured test data of naturally sedimentary soils. It is found that the coefficient of consolidation is not a constant during consolidation, behaving differently from the theoretical assumption of consolidation coefficient keeping as a constant. The fundamental issues responsible for this deviation are revealed. By introducing a normalized index, it is shown there are two changing modes of the normalized consolidation coefficient: inverse hook and slow falling, with the variations of normalized compressibility coefficient and the normalized hydraulic conductivity. It is also found that there are three change modes of the coefficient of consolidation during consolidation: a sharp decrease followed by a gradual increase, an increase, and an decrease. This study clarifies that different naturally deposited soils behave different changing behavior of the consolidation coefficient during consolidation. Even the undisturbed and remolded samples of a same naturally deposited soil behave differently changing modes. The varying modes and varying extents of consolidation coefficient during consolidation exhibit diversity, indicating difficulty in establishing an approach to compensate for the theoretical defect of assuming a constant of consolidation coefficient. It is clarified that the consolidation coefficient is a product of the analytical solution in consolidation theory. It is pointed out that the related methods derived from the theoretical assumption of the consolidation coefficient as a constant are questionable in scientific research and practical engineering.