The determination of Pb in wastewater by AAS with the use of trapping is introduced. The principle of it is that the Pb atoms condense on the surface of the stainless steel trapping tube when cooling water is passing through the tube. After a period of trapping the cooling water stops passing. Then the temperature of the trapping tube goes up swiftly , and the atom Pb which has trapped on the surface of stainless steel tube is evaporated, producing a strong absorption peak signal. Quantitative analyses can done according to the linear function of the signal and concent ration. The characteristic concentration is 3. 72×10^-3 mg / L by trapping for 4 min. The detection limit is 0. 011 mg/ L. This method can increase the sensitivity by 114 times as compared to convent ional AAS. The relative std. deviation is 0. 99 in 6 times of Pb determination.