Oxygen evolution reaction (OER) is the main limiting step of overall water splitting, and it is crucial to develop high-performance electrocatalysts for increasing the OER efficiency. Herein, a tin layer is coated on the nickel-cobalt foam surface with an electrodeposition technique and the multiple-phases of tri-metal Sn-Co-Ni phosphide (SnPi@CoP-Ni5P4/NCF) are simultaneously formed with a simple phosphating step. The synthesized SnPi@CoP-Ni5P4/NCF exhibits an excellent electrocatalytic performance for OER, which can transfer the current densities of 10 and 600 mA cm−2 (j10 and j600) in 1.0 M KOH with low overpotentials of 180 and 364 mV, respectively. The ultra-stable OER process is characterized by a low decay (2.1%) over 50 h I-t test with j100. The quantum chemistry simulations reveal the synergistic effects of the tri-metal phosphide, while the oriented crystal phases of SnPi@CoP-Ni5P4 enable the electron redistribution for lowering H2O adsorption energy and decreasing the energy barrier for dissociation. The in situ Raman results suggest that the formed NiOOH and CoOOH act as the active intermediates for OER in the alkaline medium.