Abstract: Based on the requirement that GPS geodetic height cannot be directly applied to engineering and needs to be converted to normal height, aiming at the characteristics of GPS control alignment in a bid section of South-to-North Water Transfer Project, the application of orthogonal function, cubic spline and Akima curve fitting and corresponding improved methods to GPS elevation fitting is emphatically studied. The results show that the accuracy of the three improved methods can be improved when the point selection is suitable for the linear fitting region. Among them, the accuracy of the fitting results by the improved Akima curve fitting method is improved nearly three times, and total mean square error of elevation difference reaches 6.07 mm, which shows the obvious superiority of the improved Akima curve fitting method for this selection scheme.