Abstract: The problem in the line heat source method in measuring the thermal parameter of frozen soil is put forward. The variation of unfrozen water content with respect to temperature is the primary cause for the measuring error in the heat source method. The 2-D heat conduction equation for infinite zone-point heat source is simplified as the 1-D one for central symmetry-heat flow boundary. The dual needle method is used to measure the soil samples in cutting ring at various temperatures, and the corresponding numerical simulations for heat conduction are conducted. The volumetric heat capacity due to phase change and unfrozen water content is obtained through back calculation. The results show that as the temperature of soil is higher than -0.7 ℃, the volumetric heat capacity due to phase change is much greater than the volumetric heat capacity of frozen soil, and it varies with the temperature drastically, hence the heat source method loses efficacy; as the temperature of soil is ranging from -0.7~-4 ℃, the measured error remains larger in the volumetric heat capacity. As the temperature of soil is lower than -4℃, the volumetric heat capacity due to phase change is little and steady so that the measured thermal parameters meet the requirements of accuracy to a certain degree.