Bimetallic transition metal phosphides (TMPs) as potential candidates for superior electrochemical performance are still facing great challenges in the controllable preparation of two-dimensional (2D) structures with high aspect ratio. Herein, a novel structure of quasi-monolayered NiCo-bimetal-phosphide (NiCoP) has been designed and successfully synthesized by the newly developed process combined with ultrasonic-cavitation and phase-transition. This is the first time to break through the controllable preparation of 2D bimetal-phosphides with a thickness of 0.98 nm in sub-nanoscale. Based on the advantages of 2D quasi-monolayer structure with dense crystalline-amorphous interface and the reconfigured electronic structure between Niδ+/Coδ+ and Pδ−, the optimized Ni5%CoP exhibits an outstanding bifunctional performance for electrocatalyzing both hydrogen evolution reaction and oxygen evolution reaction in an alkaline medium. Ni5%CoP presents lower overpotentials and voltage of 84 mV & 259 mV and 1.48 V at the current density of 10 mA·cm−2 for HER & OER and overall water splitting, respectively, which are superior to most other reported 2D bimetal-phosphides. This work provides a new strategy to optimize the performance of electrolytic water for bimetal-phosphates and it may be of significant value in extending the design of other ultrathin 2D structured catalysts.