Abstract: In order to investigate the affecting factors and mechanism of fly ash’s adsorption on heavy metal molybdenum, the factors of fly ash particle size, dosage, adsorption time, pH, and temperature were studied through single factor experiments with fly ash as adsorbent and ammonium molybdate as adsorbate. The optimum adsorption condition was obtained: fly ash’s particle size was 80-100 mesh, the dosage of adsorbent was 2.5 g, pH was 3, reaction time was 30 min, and the temperature was 30 ℃. Under this condition, the removal rate of ammonium molybdate by fly ash was 80%-85%.Under the optimum adsorption condition, the desorption experiment of fly ash was carried out by pure water, NaOH with the concentrations 0.01, 0.05, 0.10 mol/L and NaCl with the concentration 0.05 mol/L. The conclusion was that chemical adsorption effect was greater than the physical adsorption effect in the adsorption process of fly ash.