In order to suit the needs of the rapidly developing tourism in Hangzhou and improve the water quality of the West Lake, the Hangzhou Municipality hewed a tunnel and diverted water from the Qiantangjiang River to the West Lake to replace the original water in the West Lake.The worsening water quality of the lake is mainly attributable to the increasing organic substances and mineral matters in water, i. e., the increase in mineralization intensity of the water. The mineralization intensity of water, however, is approximately in linear relationship with its apparent resistivity. Therefore, we can use the apparent resistivity method to monitor the water-changing Work. From the equi-apparent resistivity graph of water that has been drawn, we can know the dynamic variation of changing water, i. e., the shape of mixed flow. As the concentration of substances in lake water and the variation in apparent resistivity obey the infinite dilution law, namely, the negative exponent attenuation law, we may establish a methmatical model and thus reveal the motion law of mixed flow theoretically.Based on the observation result and mathematic model, this paper also makes a tentative discussion on the economical and reasonable water-changing method.