The most common causes of pain under the second thumb are as follows: First, the hallux valgus deformity of the thumb leads to an increase in the local weight bearing of the second toe, and after long-term walking, local corns and calluses of the second thumb are likely to occur, leading to pain under the second thumb of the foot; second, the possible cause is ischemic necrosis of the second metatarsal head, which leads to pain under the second toe. Third, the possible reason is that the nerve of the toe under the second thumb has a neuroma, which will cause obvious pain after weight-bearing walking; fourth, the possible reason is that the Tsuga plate under the second thumb has been damaged, which will cause obvious pain after weight-bearing walking. Whichever the cause of the pain, you should go to the hospital and see a foot and ankle surgeon for a detailed examination and diagnosis.