Arm swelling and pain may be caused by strenuous activities of the upper limbs, deep vein thrombosis of the upper limbs, trauma and other reasons. 1. Strenuous activity of upper limb: lactic acid accumulates after strenuous activity of upper limb, and lactic acid stimulates peripheral nerves of upper limb, which causes arm distension and pain; strenuous activity of upper limb can cause damage to muscles of upper arm, which causes pain. 2. Deep vein thrombosis of the upper extremity: deep vein thrombosis of the upper extremity leads to blood reflux obstacle in the upper extremity and blood stagnation, resulting in swelling and pain in the upper arm. 3. Trauma: due to trauma such as impact on the upper arm, causing damage to the muscles, fascia, bones and other tissues of the upper arm, causing pain and inflammatory swelling, and activity will aggravate the pain. There may be other reasons for arm swelling and pain, patients should go to the hospital in time to improve the examination after eliminating physiological reasons, and give targeted treatment in time after clarifying the cause of the disease, so as to avoid further aggravation of the condition.