Single-layer DLC film on gear steel exhibits limited mechanical and tribological performance due to weak adhesion and insufficient load-bearing capacity. An effective method to improve the properties of DLC film is to strengthen the gear steel surface by introducing nitriding pretreatment. In this study, PN/DLC and P-PN/DLC duplex layers were prepared on the 30CrMnSiNi2A gear steel by a combination of plasma nitriding and magnetron sputtering, and their properties were analyzed in comparison with single-layer DLC film. The results indicated that the mechanical and tribological properties of the PN/DLC and P-PN/DLC duplex layers were significantly improved. In particular, the bonding strength and hardness of the PN/DLC duplex layer increased by 46 % and 43 %, respectively, compared to the single-layer DLC film, due to the reinforced gear steel surface and mechanical interlocking. Furthermore, the polishing treatment of the nitrided layer is crucial, as it eliminates surface micro-convexities, greatly reduces roughness, suppresses the ploughing effect during the friction process, and facilitates a change in the wear mechanism from abrasive wear to oxidative wear. This results in the optimal tribological performance of the P-PN/DLC duplex layer, with a coefficient of friction of about 0.12, and a wear rate of only 1.44 × 10 −8 mm 3/(N·m) - approximately 57.5 % lower than that of the single-layer DLC film. Obviously, the strategy of preparing duplex layers by nitriding and magnetron sputtering provides a practical surface modification method for high-performance gear systems.