Poisson-Nernst-Planck (PNP) equations model the diffusion of ions under an electric field induced by charged ions themselves. If they are coupled with Navier-Stokes equations, ions influence the fluid through an induced force, and the fluid advects ions in return, and we get the so-called Poisson-Nernst-Planck-Navier-Stokes (PNP-NS) equations which govern the dynamics of concentration of positively charged ions, concentration of negatively charged ions, electrostatic potential, fluid velocity, and fluid static pressure. The total energy of the PNPNS equations is composed of the chemical energy, the electric energy, and the kinetic energy of the fluid. In this talk, we introduce a new method that preserves this energy law exactly at the discrete level. The method is a mixed finite element method and uses no additional sophisticated techniques.