The structure of sodium sulfate saline soil is prone to change with seasonal temperature, which is detrimental to projects. To study the freezing temperature characteristics of sodium sulfate saline soil under unidirectional freezing condition, we carried out unidirectional freezing test by using a controlled temperature freezer on indoor-prepared samples with initial moisture content of 15%, 17%, 19% and 21% and salt content of 0%, 1%, 2%, 3% and 5%. Results manifest that: 1) The time required to reach freezing temperature differs slightly at different locations in the same soil despite the same freezing temperature. 2) The initial moisture content and salinity of sulfate saline soil has a remarkable influence on freezing temperature. When the salinity of sodium sulfate is 1% and 2%, the freezing temperature first changes sharply but then gently with the increase of initial water content; the maximum freezing temperature is reached at optimal water content, and afterwards, the curve of freezing temperature tends to be flat with the change of initial water content. When salt content is high (5%), the freezing temperature rises linearly with the increase of initial water content. 3) The freezing temperature of sodium sulfate saline soil declines with the increase of salt content. When salt content is 1%-2%, the freezing temperature of soil with different water content and the time required to reach freezing temperature all presents a trend of “normalization”. Sodium sulfate solution should be regarded as a non-ideal dilute solution in the calculation of freezing temperature of sodium sulfate saline soil. In addition, at the same initial water content, the relationship between sodium sulfate salt content and freezing temperature can be well fitted by cubic polynomial.