An approach was devised to quantify free water content in the leaves of sawtooth oak (Quercus acutissima
Carruth.) under different states using differential thermal analysis. The freezing temperatures, peak heights of the exotherms and the areas under the exotherms were clearly observed in the time-domain scanning curve of the differential thermal analysis during cooling, while the larger areas under the exotherms were seen in the temperature-domain scanning curve of the differential thermal analysis, providing a higher sensitivity in quantifying the free water content in the samples. The boundary between free and bound water in the tissues was (−8.23 ± 0.21) MPa, close to the osmotic potential of a 2 mol/kg NaCl solution (−8.71 MPa). Freeze-killed leaves demonstrated uniform solution freezing features with sharp, narrow peaks and higher peak values (1.91 times that of living leaves) attributed to rapid temperature changes. In living leaves, the peaks were broader, and values were lower, suggesting notable delays in water phase change within cellular membranes. Thus, differential thermal analysis could effectively determine the phase change parameters of free water in plant tissues during freeze-thaw cycles and quantify the free water content in plant tissues.