Objective    This study aimed to provide anatomical evidence to solve the flexion deformity of the interphalangeal joint of the thumb caused by the surgical procedure of reconstruction of thumb opposition function with pectoralis minor muscle.　Methods　On 15 cadaveric hand specimens with the thumb in the opposition position, we measured the natural flexion angle of the interphalangeal joint of the thumb before and after transection of abductor pollicis brevis; Meanwhile, the interphalangeal joint was rendered into flexion angle of 45°and 60°by external force to analyze the effect of abductor pollicis brevis on the flexion and extension function of the interphalangeal joint of the thumb.　Results　When the thumb was in the opposition position, the natural flexion angles of the interphalangeal joint of the thumb was （13.30±2.13）°and （24.03±1.25）°before and after transection of abductor pollicis brevis; rendering of the interphalangeal joint of the thumb at the flexion angle of 45°required application of external force by（0.50±0.08）N and（0.22±0.07）N, respectively, while rendering of the interphalangeal joint at 60°required application of external force by（1.48±0.09）N and（1.15±0.04）N, respectively.　Conclusions　The main function of abductor pollicis brevis is to dorsally extend the interphalangeal joint of the thumb. Consequently, the function of abductor pollicis brevis on the thumb should be restored when reconstruction of thumb opposition with functional pectoralis minor muscle flap is performed，so that we can rectify the postoperative the flexion deformity of the interphalangeal joint of the thumb.