The first thing to determine is whether the urine output in renal failure is acute renal failure or chronic renal failure. The recovery period of acute renal failure is generally divided into oliguric and polyuric phases. In the oliguric phase, the urine output is less than 400ml per 24 hours, and in the polyuric phase, the urine output is more than 2500ml per 24 hours, up to 8000-1000ml. in chronic renal failure, the urine output generally decreases gradually, with different differences according to the patient’s primary disease. for example, in early chronic renal failure, nocturia can occur, which is a manifestation of impaired renal tubular function. Increased nocturia means that the number of nocturnal arisings per night is greater than 3, or the nocturnal urine volume is greater than the daytime urine volume. Later on, the urine output will gradually decrease, and even anuria will occur. When the urine volume in 24 hours is less than 100ml, it is called anuria. Therefore, in chronic renal failure and anuria, it is necessary to strictly control the amount of water intake and perform renal replacement therapy in time to expel the toxins from the body.