Objective To explore the effect of substrate bias on the quality of Ti/ TiN film. Methods Making use of the technology of multi-arc ion plating, magnesium alloy was plated with Ti followed by TiN plating under different bias voltage. The morphology of films was observed by SEM, the adhesion of the film with substrate was investigated by scratch tester, and the corrosion resistance of film samples plated with different bias was compared to that of AZ31 magnesium alloy through electrochemical working station. Results When the bias voltage was 200 V, the TiN film was uniform and compact, the velocity of film formation was quick, and the corrosion resistance of the film was the best. When the bias voltage was 200 V, the film had the strongest bonding with the substrate and considerable thickness, besides, the corrosion resistance was relatively high. Conclusion Bias voltage during Ti plating of film had significant impact on the quality of the subsequent TiN plating, and the coating quality was the best when the bias voltage was 200 V, the resulting film was compact, the velocity of film formation was fast, and the film had excellent corrosion resistance.