Cobalt phosphides (CoP_x) as a representative of transition metal phosphide (TMP) catalysts show great potential for highly-efficient electrocatalytic water splitting for H₂ production. However, the current synthetic strategies of CoP_x are complex and time-consuming. Herein, a novel, facile and fast molten salt strategy for one-step synthesis of CoP_x (CoP/Co₂P) was first proposed. Moreover, the molten salt's strong polarization force endows CoP_x with abundant phosphorus vacancy (PV) defects. The synthesized CoP_x show impressive catalytic activities towards both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline conditions. This enables the overall water splitting to reach the current densities of 10 and 100 mA cm⁻² driven by only 1.75 and 1.90 V voltage, respectively. Mechanism investigation reveals that PV defects lead to the optimization of the electronic structure, decreased energy barriers of intermediates’ formation, and enhanced electronic conductivity, all of which boost the electrocatalytic activity. This study provides a paradigm of using molten salt for the synthesis of advanced TMPs, which not only simplifies the synthetic procedures, but also provides insight into defect engineering that involves the use of molten salt medium to obtain a better activity.