In the determination of soil fluorine on the loess area the method of alkali melting-ion opting eletrode, compared with colorimetric analysis by fluorine reagent and by atizarin-zirconium sulphonate, titration by the thorium nitrate, is with the characters of wider measuring range of 0.05-1, 900mg/l, better selection, simple and faster determination, etc..The results obtained with the method show that the rate of recovery is 93-101%, the variation coefficient is mostly less than 5%. The comparison with fluorine content of standard sample demonstrates that the determined results obtained with alkali melting-ion opting electrode are accurater and preciser.