The population size of freshwater leech Whitmania laevis had been observed every month in Guangzhou kaleyard slot from 1994 to 1998, and ten chemical factors of water were measured. The relationship between the population dynamic of W. laevis and chemical factors was studied by stepwise regression analysis method. The results showed that the population size dynamic of W. laevis was different every year. The maximum of population size appeared in April, May, and June, and the total population size of three months was accounted for 57.14-71.18% of whole year. The major chemical factors that effected on the annual population size were different every year, and the factors included pH value, PO(4)3-, Na+, Ca2+, and Mg2+. Fe3+ was the chief chemical factor effecting on population size during the 5 years observation. The behavior of leech W. laevis was also described in this paper.