In order to improve the laser absorption rate of copper, a method of copper laser welding assisted by the surface pretreated by the nanosecond laser direct writing was proposed. Nanosecond laser was used to write directly on the surface of copper to produce period microstructure covered by nanoparticles. The pretreated surface could effectively absorb the fiber laser and improve the laser absorption rate to ensure a smooth laser welding of copper. The mechanism of the pretreated surface reducing the reflection of copper was illustrated, the influence of welding process parameters on the weld formation was studied, and the microstructure and mechanical properties of the weld were analyzed. The results showed that surface nano-particles can effectively reduce the laser reflectivity of copper. With the increase of welding laser power, the weld appearance altered from a shape of wide- and-shallow to narrow-and-deep shape. The microstructure was dendritic crystals based on copper grain at the boundary of bond line at partial melting region, and the tails of dendritic crystals were curved towards the welding direction, instead of growing toward the axis of weld, and the bunches of dendritic crystals were uniform in thickness. Line scanning images showed that the treatment of laser direct writing barely had any effect on the metallurgical behavior of the copper welds, and there was only a small amount of oxygen in the range of 53 μm from the weld surface. Besides the single-phase solid solution of copper, there wasn't any heterogeneous such as oxygen introduced in the weld. The tensile strength of the joint was 81.4% of that of the base metal, and the hardness of the weld center was 65.8HV0.1. The fracture was ductile, and the weld remained certain plasticity.