In this paper the optimal conditions for the method of DTPA extraction - phenanthroline colorimetry were studied to measure the content of soil available iron. The results showed that the content of soil available iron could be determined at 510 nm by using phenanthroline as the chromogenic reagent. The optimal measuring conditions were: 1 ml of 100 g/L hydroxylamine hydrochloride, 8 ml of 100 g/L ammoniom acetate and 10 ml of 100 g/L phenanthroline colorimetry added into 5 ml of the measured sample under the temperature of 60℃ for 30 min coloration. The relative standard deviation of this method was less than 2.0% and recovery rates ranged from 98.6% to 100.2%. It suggested that the method is simple, fast, stable, high sensitive and fits for the alkaline or alkaline soil.