The work aims to investigate the effect of the Ti/Al transition layer on composition, bonding structure, mechanical properties, and adhesive strength of Ti/Al co-doped DLC films (Ti/Al-DLC). Ti/Al-DLC film with Ti/Al transition layer was deposited on 316L substrates by novel linear ion beam source composited with DC magnetron sputtering process. The interface morphologies and steady structuresof the films were analyzed by field emission scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), and confocal laser Raman spec-troscopy. In-depth composition analysis of the sample was performed by glow discharge spectrometer, and the nano-indentation, scratch tester system, and residual stress meter were used to evaluate the hardness and elastic modulus, adhesive strength, and internal stress, respectively. Compared to the film without transition layer, the film with Ti/Al transition layer was slightly affected in the structure and mechanical properties and the optimal mechanical properties were achieved at the sputtering current of 2.5 A. When the sputtering current was 2.5 A, the addition of a Ti/Al transition layer increased the adhesive strength from 54.5 N to 67.2 N, which increased by 19% compared to the film without transition layer. Moreover, the residual stress decreased from 1.28 GPa to 0.25 GPa, which decreased by 80% significantly. The Ti/Al transition layer can further reduce the difference in lattice strain between the DLC film and the substrate and the high interfacial stress caused by the different expansion coefficients. The typical columnar crystal structure for the transition layer formed between the film and the substrate can promote the mechanical interaction between the film-based interfaces and significantly improve the adhesion between the film and the substrate without damaging the mechanical properties.