Objective To investigate the biomechanical effects of interbody cage height on cervical spine during anterior cervical discectomy and fusion (ACDF) surgery, so as to provide references for selection of interbody cage. Methods The finite element model of normal cervical spine (C2-7) was built and validated, and the cages with different height (5, 6, 7, 8 mm) were implanted into C5-6 disc (cage5, 6, 7, 8 model). All the models were loaded with pure moment of 1.5 N?m to produce flexion, extension, blending and axial torsion motions on the cervical spine, and the effects of cage height on range of motion (ROM), facet joint stress, intervertebral pressure in cervical spine were investigated. Results The intervertebral angle at the fusion segment increased by 0.68°with per 1 mm-increase of height. The ROM at C5-6 after cage implantation was less than 0.44°. The influence of cage height on ROM in C4-5 was greater than that in C6-7, and the changes of ROM in non-fusion segments were less than 7.3%. The cage height variation had a smaller impact on the facet joint stress and intervertebral pressure. The stresses in the capsular ligament, cage and screw-plate system increased gradually with the increase of cage height, and these stresses in the cage6, 7, 8 models were much higher than those in the cage5 model. Conclusions For patients who need implanting fusion cage, the cage height should be 0-1 mm greater than the original intervertebral space height.