Abstract: Due to alkaline earth metal oxide such as calcium and magnesium being existed in the fly ash, its slurry is alkaline. It’s easy to pollute the underground water when the slurry was filled in the goaf. Thus, some experiments such as composition test, slurry pH characteristic test, carbon and alkali reduction test were carried out by using fly ash selected from Fugu Power Plant in northern Shaanxi Mining area. Based on the theoretical relationship between slurry pH value and OH⁻ concentration, the reaction process of carbon and alkali reduction of fly ash was divided into stages and a two-stage coupling method for high efficiency mineralization was proposed. The results showed that the slurry of fly ash had high alkali for the reason that the alkali metal oxides such as CaO,MgO,K₂O,etc being existed and the pH value increased with the slurry concentration. While the pH value was not affected by the concentration when the slurry concentration was more than 30%. At the same time, the OH⁻ concentration was saturated after fly ash being dissolved in water for 20 minutes due to the fast reaction of alkali earth metal oxides with water. And the reaction product is calcium carbonate of calcite type as well as 20 grams of carbon dioxide being mineralized per kilogram of fly ash. In the process of carbon fixation and alkali reduction, pH reduction curve reflected as reverse S type and the pH reduction rate was divided into three stages, referring to slow, fast and slow. The changing points of three stages were that pH values being 11.39 and 7−8 as while as the first stage cannot be eliminated. Simultaneously, the pH value and alkali reduction were not the same concept for alkali reduction referring to the decrease of OH⁻concentration in the slurry. So the first stage corresponded to the rapid alkali reduction and the second and third stage corresponded to the deep alkali reduction. The carbon fixation amount was mainly determined by the first stage, not the second stage of rapid pH value decrease for the reason thar the CO₂ utilization rate of the first stage was about 30.78%, while the total CO₂ utilization rate of the second and third stages was about 9.04%. Based on the stage division of reaction as while as the difference of alkali reduction rate and reaction device volume, a two-stage coupling method of high efficiency mineralization was proposed. The research results have great significance for analyzing the mechanism of carbon fixation and alkali reduction of fly ash, improving the efficiency and promoting the industrial application of fly ash disposal.