The on-line detection technology of heavy metals in surface water was established by the on-line preconcentration and the on-line thermal digestion method based on microwave plasma atomic emission spectrometry（MP-AES）.The heavy metal elements（Cd,Cu,Cr,Ni,Pb,Fe,Mn and Zn） in water samples were in situ monitored simultaneously in Xijiang River,one of the Pearl River main streams.The results showed that the on line detection technology could meet the requirements of surface water environment quality standard（GB 3838-2002） for quantitative detection of those heavy metals.According to the analytical results of heavy metal elements in the environmental standard samples,the determination results were consistent with the standard values.The recoveries of spiked water samples ranged from 81.5%to 102%.The detection limits of this detection technology for heavy metals ranged from 1.14 μg/L to 5.34 μg/L,and the RSDs of the detection results were in the range of 0.79%-9.4%.The method could meet the requirements for rapid,continuous and accurate monitoring of heavy metal element contents in surface water.