The invention provides a preparation method of an environment-friendly sewage treatment agent. The preparation method includes: using ferrous sulfate, diatomite, modified lignin sulfonate, polyferric chloride, activated carbon, sodium carbonate, cottonseed hull, polyacrylamide, activated sludge and water as raw materials; adding ferrous sulfate, diatomite, modified lignin sulfonate, polyferric chloride, activated carbon, sodium carbonate and cottonseed hull into a grinder for grinding, mixing well, adding activated sludge and water while stirring, heating to 35-40 DEG C after reaction for 20-30 min, adding polyacrylamide while stirring, and stirring and mixing well for reaction for 1-5 h. The preparation method is simple and free of secondary pollution, and the treatment agent prepared by the method is high in removal rate of BOD, COD and suspended matter in sewage and wide in application range.