Solving the frequency of Doppler signal is the foundation for speed measurement of high-speed moving target. When measuring speed of the moving projectile by extremum, the extreme value point carries instantaneous frequency information of the Doppler signal. So it is very important to find the location of extreme value point when estimating the frequency of projectile echo signal accurately. The measurement precision of traditional extremum method is more restricted. A method of quadratic curve fitting is proposed to deal with the noise of echo signal, all the extreme value points are found out according to the principle of quadratic function, and invalid extreme value points produced by the piecewise fitting method are eliminated. Through the analysis of actually measured projectile echo signal, the accuracy for determining the position of extreme value point is improved and foundation for high precision measurement of moving targets is laid.