Exploring earth-abundant and effective electrode material for electrocatalytic water splitting is vital for energy conversion devices. In this study, S doped NiCoP nanosheet arrays with rich P vacancies (A-NiCoP vertical bar S) are developed through plasma treatment. Starting from growing NiCoP through hydrothermal and phosphating processes, S dopant and P vacancy are simultaneously introduced into NiCoP by Ar plasma treatment. The synergic effect of S dopant and P vacancy leads to increased exposed active sites, optimized surface chemical states and improved electrical conductivity. Consequently, A-NiCoP vertical bar S shows superior activities with low overpotentials of 88 mV at 10 mA cm(-2) for hydrogen evolution reaction (HER) in 1 M KOH, as well as a low Tafel slope of 73.7 mV dec(-1). Further, theoretical calculations demonstrate that the synergistic effects of S dopant and P vacancy can modify the electronic structure and lower free energy of hydrogen adsorption in NiCoP, thus promoting the reaction mechanism. Moreover, the water-splitting configuration by A-NiCoP vertical bar S only needs 1.60 V to attain 10 mA cm(-2). (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.