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The unfrozen water content (UWC) of rocks at low temperature is an important index for evaluating the stability of the rock engineering in cold regions and artificial freezing engineering. This study addresses a new method to estimate the UWC of saturated sandstones at low temperature by using the ultrasonic velocity. Ultrasonic velocity variations can be divided into the normal temperature stage (20 to 0 ℃), quick phase transition stage (0 to −5 ℃) and slow phase transition stage (−5 to −25 ℃). Most increment of ultrasonic velocity is completed in the quick phase transition stage and then turns to be almost a constant in the slow phase transition stage. In addition, the UWC is also measured by using nuclear magnetic resonance (NMR) technology. It is validated that the ultrasonic velocity and UWC have a similar change law against freezing and thawing temperatures. The WE (weighted equation) model is appropriate to estimate the UWC of saturated sandstones, in which the parameters have been accurately determined rather than by data fitting. In addition, a linear relationship between UWC and ultrasonic velocity is built based on pore ice crystallization theory. It is evidenced that this linear function can be adopted to estimate the UWC at any freezing temperature by using P-wave velocity, which is simple, practical, and accurate enough compared with the WE model.