Objective To provide the anatomical basis of contralateral C7 root transfer for the recovery of the forearm flexor function. Methods Thirty sides of adult anti-corrosion specimens were used to measure the length from the end of nerves dominating forearm flexor to the anastomotic stoma of contralateral C7 nerve when contralateral C7 nerve transfer was used for repair of brachial plexus lower trunk and medial cord injuries. The muscle and nerve branches were observed. The length of C7 nerve, C7 anterior division, and C7 posterior division was measured. Results The length of C7 nerve, anterior division, and posterior division was (58.8 ± 4.2), (15.4 ± 6.7), and (8.8 ± 4.4) mm, respectively. The lengths from the anastomotic stoma to the points entering muscle were as follow: (369.4 ± 47.3) mm to palmaris longus, (390.5 ± 38.8) mm (median nerve dominate) and (413.6 ± 47.4) mm (anterior interosseous nerve dominate) to the flexor digitorum superficialis, (346.2 ± 22.3) mm (median nerve dominate) and (408.2 ± 23.9) mm (anterior interosseous nerve dominate) to the flexor digitorum profundus of the index and the middle fingers, (344.2 ± 27.2) mm to the flexor digitorum profundus of the little and the ring fingers, (392.5 ± 29.2) mm (median nerve dominate) and (420.5 ± 37.1) mm (anterior interosseous nerve dominate) to the flexor pollicis longus, and (548.7 ± 30.0) mm to the starting point of the deep branch of ulnar nerve. The branches of the anterior interosseous nerve reached to the flexor hallucis longus, the deep flexor of the index and the middle fingers and the pronator quadratus muscle, but its branches reached to the flexor digitorum superficials in 5 specimens (16.7%). The branches of the median nerve reached to the palmaris longus and the flexor digitorum superficial, but its branches reached to the deep flexor of the index and the middle fingers in 10 specimens (33.3%) and to flexor hallucis longus in 6 specimens (20.0%). Conclusion If sural nerve graft is used, the function of the forearm muscles will can not be restored; shortening of humerus and one nerve anastomosis are good for forearm flexor to recover function in clinical.