The ratio of urine osmolality can reflect the function of urine concentration, especially in patients with acute renal failure, and the level of urine osmolality can reflect whether the disease is prenephrogenic, nephrogenic or postnephrogenic. Urine osmolality is basically the same as urine specific gravity, and urine specific gravity is often used clinically instead of osmolality to observe the condition. If renal failure is acute nephrogenic, due to damage to renal tubules and renal interstitium, there will be a decrease in urine specific gravity <1.010. When acute renal failure, occurring in pre-renal, the effective blood volume in the body is insufficient, urine osmolality will be significantly higher than normal, generally urine specific gravity >1.020. When renal failure occurs in the history of obstructive nephropathy, such as post-renal, urine osmolality will be normal range, urine specific gravity in between 1.015 and 1.020. Therefore, the correct view of urine osmolality has obvious significance in guiding the treatment and prognosis of the disease.