The high-index facets of face-centered cubic metal have high surface energy, and the thermodynamics of crystal growth makes the high-index facets disappear during the crystal growth. The surface energy of high-index facets can be reduced through adsorption of molecules during crystal growth, and metal nanoparticles with high-index facets are thus formed. The shape-controlled synthesis of metal nanocrystals remains a big challenge even today. The shape evolution mechanism of metal nanocrystals with different facets has not yet been well elucidated. In this work, platinum nanocrystals of different shapes, octahedra with low-index facets, tetrahexahedra and concave hexoctahedron enclosed with high-index facets, were synthesized by the square-wave-potential method (SWP). The same precursor and the same adsorption molecules were used to synthesize Pt nanocrystals, but a series of parameter were varied, such as precursor concentration, growth potential, oxidative etching potential, with/without electrolyte in solution, and frequency of the SWP. This paper discusses the details about shape-controlled synthesis and proposes the preliminary mechanism of formation of Pt nanocrystals. The current study has illustrated that the electrochemical approach is an effective and facile route in tuning Pt nanocrystals' shape and corresponding properties, shedding lights on the design and preparation of Pt nanocatalysts.