In this paper, Al-Cu alloy-based Si3N4 composite was prepared by using a powder metallurgy technology. The phase, surface appearance and hardness of the composites with different Si3N4 contents were investigated. The water lubrication behaviors of Al-Cu alloy-based Si3N4 composites were evaluated, and the corresponding low friction mechanisms were explained. Experimental results show that the main structure of sintered Al-Cu alloy contains of α(Al) + θ(CuAl2) metal phase. With the addition of Si3N4, the AlN, Al2O3 and SiAlON ceramic phases appear for the Al-Cu alloy-based Si3N4 composites. The composites with 40 wt.% Si3N4 content demonstrates flat net structure on the surface with the highest hardness. Under water lubrication condition, the low friction coefficient of 0.05 can only be achieved for the composites with proper Si3N4 content. Analysis reveals that the flat contact surface and tribo-chemical reaction seem to be the two essential conditions for the low friction. The major wear mechanisms transfer from mechanically dominated wear at the running-in process to the tribo-chemical dominant at the low friction stage.