The development of transition metal phosphides (TMPs) electrocatalysts with appropriate binding strength towards intermediates and resistant to chlorine in alkaline seawater electrocatalysis remains a formidable challenge. Herein, an approach for manipulating electron redistribution in Ni2P/Ni5P4 treated by Co cationic doping engineering (denoted as Act-Co-NiPx) is proposed, exhibiting excellent catalytic activities and stability toward HER in alkaline seawater solution. First-principles calculations demonstrate that the Co doping can effectively adjust the electron redistribution of Ni2P/Ni5P4, resulting in a reduction in the hydrogen adsorption free energy for HER and an acceleration in the interfacial charge transfer. Furthermore, the enhanced Cl- adsorption energy avoids the toxicity of Cl- to the active sites in alkaline seawater splitting. Consequently, the resulting Act-Co-NiPx catalyst demonstrates remarkable HER performance with low overpotentials of 77 and 97 mV at a current density of 10 mA cm−2 in both 1.0 M KOH and 1.0 M KOH+Seasalt electrolytes. Furthermore, the seasalt electrolyzer assembled from Act-Co-NiPx achieves the low cell voltage of 1.63 V at 10 mA cm−2. This work demonstrates a feasible method for designing cost-effective and stable catalysts with excellent selectivity via regulating the electronic structure of TMPs for seawater electrolysis.