Abstract: Consolidation theory is an important component of soil mechanics, and the coefficient of consolidation is generally elaborated as a key concept 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 natural saturated 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 being constant. The fundamental issues responsible for this deviation are revealed. By introducing a normalized index, it is shown that there are two changing modes of the normalized consolidation coefficient: inverse hook and slow decreasing, with varying 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 with increasing effective stress: a sharp decrease followed by a gradual increase, an increase or a decrease. This study clarifies that different naturally deposited soils exhibit different changing behaviors of the consolidation coefficient during consolidation. Even the undisturbed and remolded samples of a same naturally deposited soil exhibit 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 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 have reliability defects in scientific research and practical engineering.