The difference between the results from the precisely predicted ephemeris and precise ephemeris is very small in the water vapor monitoring network, so the precisely predicted ephemeris can be used to detect water vapor in real time. Because the ionospheric delay has great effect on water vapor, it should be eliminated. The derivation processes of three methods (Klobuchar model, dualfr equency measurements and linearly combined double frequency measurements without ionospheric delay) are described. The effects of ionospheric delay eliminated by these three methods are compared, and the results indicate that the combined method is the best.