I don't know why I found that my blood sugar was normal and my urine sugar was 4 plus signs.

It is because the blood sugar is too high, which exceeds the renal glucose threshold (8.89 ~ 10.0 mmol/L), and the glucose filtered by glomerulus can not be completely reabsorbed by renal tubules, forming osmotic diuresis. The higher the blood sugar, the more urine sugar will be excreted, and the more urine volume will be. The urine output in 24 hours can reach 5000 ~ 10000 ml. However, in the elderly and patients with renal diseases, the renal glucose threshold is increased, and urine glucose excretion is impaired, and polyuria is not obvious when blood sugar is slightly or moderately increased.