Contracture of the flexor hallucis longus (FHL) tendon is one of the most common factors leading to hallux flexion deformity. Few cases treated by arthroscopic technology have been reported. In this article, we perform a new method to lengthen the FHL tendon under an arthroscope. We present a case of 1 patient treated with this technique and followed for 2 years. The appearance of his halluces remarkably improved at the follow-up visits, and movement was refined simultaneously. From the satisfactory effect of this operation, lengthening the FHL tendon under arthroscope is an effective method to correct flexion deformity of the hallux with minimal incisions. This technique also provides an innovative application of minimally invasive surgeries to treat clinically infrequent diseases.