The normal determining method of potassium persulfate digestion with colorimetric tubes of 50mL takes long time and consumes excessive experimental reagents,and the auther proposed to use 25mL tubes.Through comparing the vacancy,the related coefficient, the rates of recovery and reproducibility of the new method with those of normal methods,we conclude that y=1.5173x+1.20×10-3,the related coefficient is 0.9998,reproducibility is between 98.4% and 104.1%,which show no apparent differents. The new method has the advantages of high speed, good precision, low experimental cost,and low second pollution.