There are three main reasons for swollen feet when sitting for a long time: first, the lower limb venous reflux is obstructed, long-term sitting causes no muscle movement and loss of venous function, coupled with the absence of valves in the veins, venous blood cannot reflux and accumulates locally, resulting in the formation of edema due to intravascular fluid extravasation; second, long-term sitting slows down venous blood flow, easily forming deep vein thrombosis in the lower limbs, causing venous reflux obstruction and lower limb edema; third, lower limb edema caused by kidney disease and renal insufficiency should also be excluded, as should liver insufficiency caused by cirrhosis, hepatitis and other liver diseases, resulting in lower limb edema due to hypoproteinemia.