In order to improve the energy efficiency of DC motor starting, the unique rotational speed extremum trajectory in the shape of concave parabola was obtained by using variational method. Simulation verified that the energy efficiency extremum of this was the lowest. To find the optimal energy efficiency trajectory, several other starting trajectories were considered, especially the convex parabola trajectory relative to the variational extremum trajectory, which was proved to have a higher energy efficiency. The simulation also proved that the simple starting method with a constant high current could further improve the energy efficiency of DC motor starting.