Double renal pelvis without obvious dilatation is a normal state, which suggests that the renal pelvis of the kidney does not have fluid accumulation and does not have tumor. During renal ultrasound examination, if the anterior and posterior diameter of the renal pelvis is less than 0.7cm, the renal pelvis is called not obviously separated, also called not obviously dilated. If there is dilatation of the renal pelvis, it suggests that there may be hydronephrosis, or it may be caused by a tumor in the renal pelvis. If there is no obvious dilatation, it suggests that it is a normal state, neither hydronephrosis nor renal tumor, in this case, there is no need to worry too much, only need to review the ultrasound of kidney dynamically, every 3 months to half a year. If renal pelvis dilatation is detected, it must be treated positively, because long-term renal pelvis dilatation will lead to renal parenchyma pressure, thinning of renal cortex, and finally gradual decline in function, which will develop into renal failure. The most common cause of renal pelvis dilatation is kidney stones.