The spermatoid plexus is composed of many small veins (about 10-20 in total) that drain the testes, epididymis and vas deferens, and finally group into the internal spermatic vein, external spermatic vein and vas deferens vein. Varicocele mostly occurs within the circulation of the internal spermatic vein. The main cause is the obstruction of the internal spermatic vein. In recent years, the incidence of the left side is 67% to 81% as found by spermatic venography, so why is this so? So the right side blood flow seems to be smooth. (2) The left internal spermatic vein anastomoses with the left renal vein at a right angle, while the right internal spermatic vein anastomoses obliquely with the inferior vena cava, so the blood flow is relatively smooth. (3) There are several venous valves in the spermatic vein, and they can stop the backflow of blood. However, the left internal spermatic vein lacks a valve at the connection with the left renal vein, so it is easy for the blood in the left renal vein to backflow to the left internal spermatic vein, thus preventing the blood from flowing back. (4) The blood in the left internal spermatic vein crosses the spinal column and merges into the inferior vena cava, which has a long journey and can be compressed by the sigmoid colon and the internal spermatic artery that bypasses the renal vein on the way. The pulsation of the superior mesenteric artery, abdominal aorta and common skeletal artery continuously hits the left renal vein and the internal spermatic vein, which hinders the return of blood to the left internal spermatic vein. By comparing the above 4 aspects, it is easy to see that the anatomical features and abnormalities are the main reasons why the left side is prone to varicocele.