The accurate quantification of soil heat flux in the freezing process of the alpine wetland in the source area of the Yellow River has an important scientific significance for understanding the water and heat exchanges between alpine wetlands and the atmosphere. By using the field observed data collected from the Maduo climate and environment comprehensive observatory of the Chinese Academy of Sciences from May 2014 to May 2015, the characteristics of soil heat flux as the alpine wetlands froze were analyzed. The effect of the latent heat of fusion on soil heat flux was also discussed. Both the heat storage and latent heat of fusion loss from above the plate must be considered when calculating the soil heat flux at the alpine wetland using the simple measurement approach algorithm. If the latent heat of fusion is ignored, then large errors can be found. The main results are as follows. (1) After the freezing front appeared, soil heat flux at a depth below the freezing front decreases and approaches zero, the liquid water content of the soil at the depth of the freezing front decreases rapidly, and the soil below the freezing front froze. In addition, the freezing released latent heat travels upward through the soil layer where the soil heat flux plate is located and observed. As the precipitation infiltrates into the soil, thus releasing the freezing latent heat, the freezing latent heat causes the observed soil heat flux to approach zero at a depth of 5 cm. (2) During the seasonally freezing processes, upward soil heat flux at a depth of 5 cm approaches zero if there is a high temperature in the morning and at noon of the previous day. This phenomenon indicates the existence of a diurnal freezing-thawing cycle. The latent heat released by soil water can reduce the amplitude of soil temperature and keep the soil temperature near the freezing point. The diurnal freezing-thawing processes solely occur in a very shallow soil layer, making it difficult to ascertain whether the diurnal freezing-thawing cycle happened not just by using soil temperature data at a depth of 5 cm. (3) Considering the latent heat of fusion factor decreases the root mean square errors of soil heat flux between the observed and calculated values from 11.5 W m^-2 to 6.2 W m^-2. These findings can contribute towards a better understanding of the land surface processes in cold regions.