Objective For patients with foot drop gait, to design a kind of anterior ankle foot orthosis (AFO) with adjustable stiffness, so as to restore natural gait of the ankle by limiting the patients’ unusual plantar flexion to the optimum extent. Methods The minimum orthodontic moment of 10 foot drop male patients was measured by self-made experimental equipment, which could be used to select optimum material modulus of the AFO. The relationship between elastic modulus and different filling structures and filling ratio parameters was studied by tensile test. A typical patient with foot drop was selected, and the anterior AFO fitting the shape of patient’s foot was quickly made by three-dimensional (3D) printing with foot geometric data and specific filling material, filling structure and filling rate. The kinematics and surface electromyography (sEMG) of plantar flexors were tested under barefoot and wearing two kinds of AFOs, so as to verify the effect of the AFO on plantar flexion. The effectiveness of the limitation and the degree of preservation of ankle valgus and plantar flexion were discussed. Results The minimum corrective torque required for 10 male patients with foot drop was 2.16 N·m. Compared with the rigid AFO, the range of motion (ROM) of plantar flexion and valgus increased by 67.8% and 88.6% respectively with the flexible AFO. The activation of the muscles responsible for plantar flexion (soleus, medial head of gastrocnemius and lateral head of gastrocnemius) also decreased by 38.3%, 46.6% and 55.8%. Conclusions This AFO with adjustable stiffness can be used for orthosis customization of patients with foot drop, providing more effective and long-term orthosis function and potential.