The possible causes of arm swelling and hardness inside are soft tissue inflammation, deep vein thrombosis, tumors and so on. 1. Soft tissue inflammation: may be due to arm trauma or strenuous exercise, resulting in local soft tissue damage, causing soft tissue inflammation. The symptoms of pain and swelling appear, resulting in the phenomenon of arm swelling and hardness inside. 2. Deep vein thrombosis: It may be due to deep vein thrombosis in the arm area, causing venous return to be obstructed, when a large amount of venous stasis accumulates in the distal tissues, resulting in obvious swelling and hardness in the arm area. 3. Tumor: the swelling and hardness inside the arm may be benign tumors such as lipoma, fibroma and so on, or malignant tumors such as lymphoma and so on. There may be other reasons for arm swelling and hardness inside, it is recommended to go to regular hospitals in time, complete the relevant examinations, after clarifying the cause of the disease, and then carry out targeted treatment or therapy under the guidance of the doctor.