Electrochemical activation has a great significance for enhancing the energy storage capability of electrode materials. To develop high-performance phosphides electrodes, herein, NF/MnCoMn-P (NCM-P) is fabricated by two-step method, and the contribution of cyclic voltammetry (CV) activation to electrochemical performance is revealed, as well as the activation mechanism. In the initial activation stage, NCM-P nanosheets are transformed into ultrathin NiCoMn-OH nanosheets. Meanwhile, hexagonal Ni(OH)2 nanosheets are generated, which greatly enhance the fraction of Ni. With an increase of CV cycle, vertical nanosheets arrays, petal structure, cracked structure and agglomerates are produced in sequence, accompanied with the capacitance attenuation. Among these structures, vertical nanosheets arrays obtained at 180 cycles deliver the highest capacitance of 2126 F g−1 (1063 C g−1) at 1 A g−1, about three times higher than that of NCM-P, which are considered as activation terminal. The microstructure evolution confirms that the activation essence of NCM-P is the generation, reassembly, swelling and gathering process of ultrathin hydroxides nanosheets. Furthermore, the activation mechanism of NiP layer on NF framework is proposed to explain the source of hexagonal Ni(OH)2. Serving as a positive electrode in asymmetric supercapacitor, activated NCM-P also presents a superior electrochemical performance, further reflecting the promising application potential of activated trimetallic phosphides.