This paper studies the optimal control problem during the deorbiting process of a tethered nano-satellite using a short electrodynamic tether. A current regulation method based on on-off switching is derived by mathematical optimization, which achieves fast deorbiting and librational stability during the long-term deorbiting. The computation effort of the optimal control is significantly reduced by using a piecewise treatment while the whole deorbiting process is divided into segments. Then within each segment, the dynamic model of the system is simplified and the current on-off switching times are determined by solving a constraint minimization problem of a control index representing the deorbiting efficiency. Direct method using Hermite-Simpson discretization is adopted in the optimal problem solving. The optimal control is applied together with a simple on-off switching regulation based on libration amplitudes monitoring to ensure libration stability in real environment. Numerical simulations are conduced to validate the proposed control strategy while major environment perturbations and accurate environment models are considered.