Due to the high density of surface states introduced in the nanorods growth process and subsequent exposure to air, electrons will be trapped in these states and recombine with photo-generated holes, which leads to a high loss of photo-generated carriers and creates a large onset potential for carrier transport. Here, we report a simple and low-cost surface treatment method, which utilizes H3PO4 to passivate surface states in the as-grown InGaN nanorods. Using optimized surface passivation condition, surface trapping states in as-grown InGaN nanorods can be removed by eliminating surface Indium aggregation, reducing the surface oxides, and saturating the dangling bonds. Hence, carrier recombination mediated by these surface states is weakened, as well as applied bias potential required for carrier transport is lowered. The optimized H3PO4-treated nanorods photoanode shows a larger photovoltage and a smaller charge transfer resistance to the electrolyte. Resultantly, a significant negative shift of onset potential from 0.7 to 0.3 V vs. RHE for the treated photoanode is achieved. Moreover, the optimized H3PO4-treated photoanode exhibits a high photocurrent density of 18 mA/cm(2) at 1.23 V vs. RHE and a maximum applied bias photon-to-current efficiency value of 1.09%, whereas as-grown InGaN photoanode reaches only 0.64%.