Abstract: To investigate the relationship between effective preloading performance and water content in prestressed concrete under cryogenic conditions, experiments were conducted on specimens with varying water contents (2.0%, 2.5%, 3.0%, 3.5%, and 4.5%) under two typical ultralow temperature conditions: −80℃ (relatively higher) and −160℃ (lower). The test results reveal that the prestress loss rates at both cooling and uniform temperature points exhibit significant fluctuations with low water content but demonstrate an increasing trend with higher water content. Notably, the prestress loss rate increases more sharply at −160℃, compared to −80℃. The prestress loss rate under −80℃ is consistently lower than that under −160℃, with the difference between the two temperature conditions amplifying as water content increases. Under −80℃, the transient prestress loss rate during the cooling stage shows a fluctuating pattern, while it gradually increases during the constant-temperature stage. In contrast, under −160℃, the transient prestress loss rate exhibits a fluctuating-increasing trend in both cooling and constant-temperature stages. Furthermore, prestress loss continues to accumulate during the constant-temperature stage at −80℃, whereas partial stress recovery occurs at −160℃. The experimental results and fitting formulas can provide references for the safety evaluation of prestressed concrete structures in LNG storage tanks.