The hydrolytic stability of Fe(a),Fe(b) and Fe(c) in different pH values of poly-ferric-flocculants was studied by using Fe-ferron time by time complexation colorimetry.The research results showed that Fe(b) was unstable,and all Fe(b) was transformed to Fe(c) after 10-15 d placement.The content of Fe(c) tended towards stability after 10-15 d.Also,the content of Fe(a) tended towards stability after 10 d.The single crystal was synthesized by the method of direct crystallization in Fe(Ⅲ)-SO2-4 water solution at normal temperature and its structure characteristic was studied by single crystal X ray diffraction method and IR (infrared spectrophotometry).The research results showed that there was no group of Fe—OH—Fe,Fe—OH and binary ferric complexed with two hydroxyl groups in the single crystal synthesized from the ferric aqueous solution in low pH(pH was about 0.5).The form of Fe in single crystal was all Fe(Ⅲ).The chemical formula of the single crystal was Fe(H2O)6(SO4)2NH4·6H2O when the ammonia water was used as the alkalinizing agent.One reason was that with the evaporation of water,these single crystals were synthesized at pH 0.5 despite of different initial pH and different initial alkalinizing agents.Another reason was that the hydrolysis distribution of ferric saline was unstable.Therefore,it was not easy to obtain the single crystal of Fe(Ⅲ)-hydroxy complexes or Fe(Ⅲ)-polymer at low pH value.The study showed that infrared spectrophotometry and single crystal X ray diffraction method have a good prospect in the research on hydrolysis distribution of flocculants.