Efficient light harvesting is one of the key prerequisites in improving the solar conversion efficiency for photoelectrochemical water splitting. As classic semiconductors for water splitting, the solid state solution GaN:ZnO based photoanodes exhibit poor water splitting efficiency mainly limited by its light absorption. To overcome this bottleneck, here we report that phosphorus modification shifts the absorption edge of GaN:ZnO from 480 nm to the red end of 650 nm and also leads to one order of magnitude increase of the carrier concentration. Further, taking the surface phosphate groups as anchors, cobalt can be adsorbed, leading to the in situ formation of cobalt phosphate as a cocatalyst for water oxidation, which results in drastically improved photocurrent density and stability. This work highlights the significance of phosphorization treatment in extending the light harvest and changing the surface reaction kinetics for an efficient solar conversion process.