Soil moisture plays an important role in land-atmosphere interaction because it not only has effects on the water cycle between the land and atmosphere, but also the surface energy distribution. This work theoretically analyzes the equations for soil moisture change in land surface models, and shows the rationality for the equations in which the soil water potential is used to describe the soil water flow in vertically inhomogeneous soil and frozen soil. Based on a Simple Unified Soil Model (SUSM) and the Clapp-Hornberger relationship, the sensitivity of soil moisture in frozen or unfrozen soil to the vertical heterogeneity is also investigated. The results show that soil water potential and hydraulic conductivity have crucial effects on the simulation of soil moisture. The parameter B (soil porosity distribution) in the Clapp-Hornberger relationship is the most important because, when it increases, it leads to decreasing hydraulic conductivity, which results in the soil water vertical distribution. Meanwhile, increases in both the saturated soil water potential and parameter B also lead to a higher absolute value of soil water potential. When soil begins to freeze, this effect would delay soil water from freezing (melting), meaning the soil temperature cannot persistently decrease (increase) due to a lack of release (absorption) of latent heat fluxes originating from the soil freezing (melting) process. Therefore, the amplitude of soil temperature when freezing or melting is larger than observed. The results of this study reveal the importance of soil heterogeneity and the use of effective parameters for soil characteristics in land surface models.