Three methods, sodium hydroxide hydrothermal treatment, sodium carbonate calcination-hydrothermal treatment, and sodium hydroxide calcination-hydrothermal treatment, were used to modify fly ash as an adsorbent for wastewater treatment. Results from XRD analysis show that the fly ash with sodium hydroxide hydrothermal treatment the fly ash has a porous structure in which alumina disappear and silica is dissolved. With sodium carbonate calcination-hydrothermal treatment exhibits nepheline and Na7Al7SiO16 structures. With sodium hydroxide calcination-hydrothermal treatment a zeolite-like new phase and porous structure can be observed. Results from SEM and BET indicate that the specific surface area increases from 0.160 m2/g for the raw material to 7.22 m2/g, 1.05m2/g and 276 m2/g for the respectively modified by sodium hydroxide hydrothermal treatment, sodium carbonate calcination-hydrothermal treatment and sodium hydroxide calcination-hydrothermal treatment. The three modified fly ash were used to treat a concentration of 80 mg/L of ammonia-nitrogen wastewater. The removal capacity for the three modified ash was 58%, 44% and 94% respectively that was significant higher than 14% for the original fly ash. These results show that fly ash by alkali modification can improve the activity and enhance the ability of ammonia-nitrogen wastewater treatment.