In consideration of application prospect of non-noble metallic materials catalysts, the study of exploring more highly effective electrocatalysts has been focused on by researchers. Herein, a novel strategy is employed to construct a heterojunction consisting of metal phosphide Ni_xFe_yP and layered double hydroxide (LDH) with graphene oxide (GO) as conductive support. By adjusting the molar ratio of Ni to Fe, a series of heterojunctions with mixed valence state Fe^δ+/Fe³⁺ and Ni^δ+/Ni²⁺ (δ is likely close to 0) redox couples are achieved and strong synergistic effects towards overall water splitting performance are found. The optimized catalyst with a Ni/Fe molar ratio of 0.72:0.33, namely Ni_0.7Fe_0.3P/LDH/GO, delivers ultra-low overpotentials for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of 79 and 198 mV at the current density of 10 mA·cm^-2, respectively. Furthermore, for overall water-splitting practical application, it only requires 1.526 V at 10 mA·cm^-2 with robust stability, which is superior to most reported electrocatalysts. Experimental results demonstrate the improved electronic conductivity, enlarged electrochemically active area and accelerated kinetics together account for the enhanced performance. This work supplies new prospects for the promotion and application of such heterojunction electrocatalysts in overall water splitting.