Because of the use of binary mixtures ammonia and water as the working fluid in STEC, if the composition of ammonia is not high enough, the working fluid may condense in the turbine, which results in a fatal damage of turbine. Since controlled variable of vapor composition has a strong nonlinear relationship with state variables vapor pressure and temperature and such relationship can not be described in mathematical models, the model free control method is preferred in the STEC control system. In this paper, a new control method of model free control based on gradient estimation is proposed. Based on the embedded property of the dynamic system, an optimal control algorithm is proposed. The robust convergence of the proposed control algorithm has been proved and the static control error with the proposed method is also analyzed. The application results of the STEC composition control have verified its feasibility and effectiveness.