A small cyst of the left kidney is a small liquid dark area in the left renal parenchyma, the main component is cellular fluid, the edges are relatively smooth, and there is no substantial lesion inside the cyst, so it is a cyst. Small cyst in the left kidney is a benign lesion, mainly due to the cystic expansion of kidney cells, resulting in the formation of cysts without other substantial tissues inside. It needs to be distinguished from kidney cancer, which is a substantial lesion with irregular edges and blood flow signal inside, while cyst is without blood flow signal. If ultrasound is not able to confirm the diagnosis, the diagnosis can be confirmed by CT scan with enhancement.