Aiming at the high precise and real-time computation of the satellite orbit in GNSS (Global Navigation Satellite System) simulator with satellite orbit, a fitting method based on Powell optimization theory is proposed. The method converts the fitting method for satellite position to the non-restraint least method, and the parameters of the satellite orbit model were obtained by the optimization theory, with which it is easy to compute the velocity, acceleration and so on. The computation examples show that, in the case of fitting the satellite ephemeris, the new method speeds up the computation three times faster than the direct ephemeris calculation, and the position error is less than 1×10^-4m and the velocity error 1×10^-6m/s. In the case of fitting the GPS precise ephemeris, the new method increases precisely two times efficiency than that of the ephemeris Lagrange interpolation.