Abstract: The study sets up the method to determine thiocyanate in wastewater by ferric salt colorimetric method.The influencing factors such as color-developing agent types and dosage,color-developing agent dilution solvent,acidity,stabilizer types and dosage,chromogenic reaction illumination,and time are optimized in condition tests,successfully solving the poor stability of the ferric salt colorimetric method.The precision of the method is all below 1.50 % and the recovery rate of reference materials is 98.00 %-102.83 %.