Phase-tunable mesoporous nickel phosphides are promising electrocatalysts for alkaline hydrogen evolution. Their catalytic behavior can be regulated by Ni/P stoichiometry and accessible mesoporous architectures. However, simultaneously controlling phosphide phase evolution while maintaining an open mesoporous framework remains challenging, as high-temperature phosphidation often causes framework shrinkage, pore collapse, and uncontrolled phase transformation. Herein, we report a charge-mediated cooperative assembly strategy for the one-pot synthesis of mesoporous Ni_xP_y with tunable crystalline phases. In this system, CTAC-derived CTA⁺ micelles, phosphate species, and Ni–citrate complexes cooperatively assemble into inorganic–organic composite micelle precursors through electrostatic interactions and hydrogen bonding. Dynamic light scattering, zeta-potential analysis, and Fourier transform infrared spectroscopy support an S⁺X⁻I-type cooperative assembly pathway, in which CTA⁺ micelles, phosphate species, and Ni–citrate complexes act as structure-directing agents, charge-mediating/hydrogen-bonding linkers, and inorganic precursor units, respectively. By regulating the NH₄H₂PO₄ dosage, the effective phosphorus supply can be precisely adjusted, enabling a stepwise phase evolution from Ni₃P to Ni₁₂P₅ and ultimately to Ni₂P while preserving the well-defined mesoporous framework. The optimized mesoporous Ni₂P-1.0 catalyst requires an overpotential of 48 mV to reach 10 mA cm⁻², exhibits a Tafel slope of 67 mV dec⁻¹, and maintains stable operation for 100 h in 1.0 M KOH. These findings establish a practical route for coupling phase regulation with mesostructure engineering in transition-metal phosphides and offer guidance for designing efficient noble-metal-free electrocatalysts toward alkaline hydrogen evolution.