Pumped storage power station has an important role in regulating frequency and peaking of large power grids. It has fast response and is climbing fast. It not only adjusts the frequency in normal operation but also provides emergency support as grid accident spare. The best scheme for under frequency pump shedding under the environment of extremely low frequency in power system is studied when it is short of large power. On this basis， a shedding pumps scheme is proposed based on optimal trajectory curve of frequency. Through establishing objective function of optimal trajectory curve of power system frequency， it uses Lagrange optimization algorithm to get the best action value of frequency in pump shedding. The frequency recovery effects of different schemes are analyzed through the PSASP software package and the sufficient evidence is given to illustrate the effectiveness and superiority of this algorithm.