There are three main reasons for swollen feet when sitting for long periods of time: firstly, lower limb venous reflux is obstructed. Long-term sitting causes muscle inactivity and loss of venous function, coupled with the absence of valves in the veins, venous blood cannot reflux and accumulates locally, leading to extravasation of intravascular fluid to form oedema; secondly, long-term sitting slows down venous blood flow, easily forming deep vein thrombosis in the lower limbs, causing venous reflux to be obstructed and lower limb oedema; thirdly, lower limb oedema caused by kidney disease and renal insufficiency should also be ruled out, as should liver insufficiency caused by cirrhosis, hepatitis and other liver diseases, causing hypoproteinemia and lower limb oedema.