Abstract: The main effect on the GPS signals of ionosphere and ionospheric refractive model are researched in this paper; then the dual-frequency corrected model of ionospheric refractive error is concluded. Aiming at the third frequency added in GPS Modernization, the triple-frequency corrected model of ionospheric refractive error and the ionosphere-free combination model of triple-frequency is deduced. The second-order ionospheric effect can be corrected and the positing accuracy can be increased through these models. At the same time, these models supply the strong support in other GPS techniques such as separating error sources in GPS, cycle slip detection and etc.