Excessive discharge of phosphate into water bodies may cause eutrophication and affect the natural water quality. The adsorption behavior of phosphate from aqueous solution on fly ash was investigated by a medium calcium fly ash in weak acid solution. Experimental findings showed that removal of phosphate was pH-dependent with the maximum adsorption capacity at the initial solution pH of 5.0-7.0. The increment in the initial phosphate concentration and temperature enhanced the adsorption capacity. Kinetic studies of phosphate followed the pseudo-second-order model. The predominant removal mechanism was ligand exchange reaction on the basis of desorption experiment. The adsorbed phosphate was difficult to leach to the environment from the waste fly ash.