The competitive phase selection during cooling a metastable alloy is mainly controlled by the process of nucleation and growth for either the equilibrium stable phase or metastable phase. However, the control of competitive phase selection is still a standing challenge. In this work, a general method for controlling the competitive phase selection process by in-situ forming nanoparticles is reported. The solidification experiments of Cu-Cr metastable immiscible alloys were performed to probe the effect of in-situ TiB2, Cr3C2/Cr7C3 or TiC nanoparticles. The results indicate that the Cr-rich minority phase originates from the liquid-solid phase transformation and exhibits coarse dendrite morphology for the Cu-Cr alloy without nanoparticles. In contrast, suitable nanoparticles may lower the Gibbs free energy barrier for the formation of metastable phase, trigger the liquid-liquid phase transformation, thus the Cr-rich minority phase almost shows spherical morphology and the size is only a few microns, even hundreds of nanometers. Furthermore, the microhardness of Cu-Cr alloys with nanoparticles is improved significantly. This strategy may have extensive application prospects for the modification of phase selection behaviors and solidification process of Cu-Cr metastable immiscible alloy.