Chemical dephosphorizations by four kinds of agents for secondary biochemical effluent were carried out and dosing experience coefficient was investigated.The results showed that when pH=7.5,FeCl₃ dosage is 6.5 mg/L,and pH=7.5,Al₂(SO₄)₃ dosage is 3.75 mg/L,the effluent TP is less than 0.5 mg/L.These two flocculants possessed the advantages of good effect,low cost and easy operation.In addition,FeCl₃ was able to reduce COD effectively with TP removal simultaneously.Therefore,FeCl₃ can be used to treat the secondary biochemical effluent which TP and COD level are impermissibly high.The method of dosing experience coefficient can calculate the quantity of reagent added by the phosphorus concentration of inflow and effluent.It is very useful for phosphorus removing projects.