Copper nanoparticles are synthesised and joined on plastic substrate in one single pass of laser with laser direct-writing technique. This direct writing process with low power laser can reduce copper ions in thin films to form copper nanomaterials and spontaneously interconnect them to gain good conductivity. The electrical performance and chemical composition of flexible electrodes can be tuned by laser power, scanning speed, and defocus distance. A mechanism of in-situ reduction and interconnection of copper nanomaterials during laser direct writing has been proposed. This method could reduce oxidation issue largely by avoiding synthesis and sintering processes of copper nanomaterials. These as-written copper electrodes are with good stability and have potential applications in flexible electronics, such as flexible heater, antenna or sensors, as demonstrated.