Abstract: The high precision data of S concentration in top soil is an important proxy to estimate the sulphur loss in top soil. A discussion is presented in this paper on the effect factors of available sulphur determination in weak alkaline soil with pH=8.5 sodium bicarbonate extractants. A further study was made on the oxidizing discoloring of extractive solution and the acidity of turbidimetry solution during the determination process. The results indicated the reliable data were obtained in the temperature range of 20℃ to 24℃, but the analytical data were higer if the temperature was higher than 30℃. The recovery of this method was 96.6%-104.6%,the standard deviation was 0.110-0.271 mg/kg, and a relative standard deviation of <5% if the S concentration is between 6-25 mg/kg in top soil. The method has the advantage of being fast, simple, accurate and highly reproducible, which could apply to determine S concentration for various kinds of weak alkaline soils.