Photocatalytic hydrogen evolution via water splitting is considered as one of the most promising sustainable energy supply strategies. The cocatalyst loading method is widely applied to overcome the inefficient charge separation and sluggish surface hydrogen evolution reaction (HER) of photocatalysts. Herein, we prepared the Ru-doped NiFe layered double hydroxide (NiFe-LDH) as a cocatalyst for g-CN. We found that the photocatalytic HER activity of g-CN is obviously enhanced by the loading of Ru-doped NiFe-LDH, the H evolution rate reaches 32 µmol/h about 5 times higher than original NiFe-LDH loaded one. Our experimental and theoretical studies reveal that the synergistic effect of NiFe-LDH matrix and Ru dopant is the key reason for the improved photocatalytic activity, where the NiFe-LDH matrix promotes charge separation and transfer, the Ru dopant synergistically lowers the energy barrier for surface H evolution reaction, then achieving excellent photocatalytic H evolution activity.