Abstract: A water pollution accident of Leptomitus lacteus took place in the Hailang River basin in February 2006. Small amounts of Leptomitus lacteus are still found in the Hejia Dam reaches of the Hailang River basin in the high-incidence period from January to February each year. In order to prevent another large scale outbreak of Leptomitus lacteus growth, a study was carried out on the Leptomitus lacteus growth change and its influencing factors in the Hailang River basin during a high-incidence period using the method of microscope observation and water quality monitoring. The results showed that Leptomitus lacteus in reduced constriction form emerged in the last third of January, and became the predominant microbial symbiont about nine days later. Leptomitus lacteus was no longer predominant after another 21 days, i.e. at the end of February. The phase of the reduced constriction form of Leptomitus lacteus emergence lasted about one month from 26 January to 24 February. This phase was the key period for Leptomitus lacteus water pollution prevention and control. The appearance of Leptomitus lacteus in the Hejia Dam reaches of the Hailang River from January to February was related to such factors as dissolved oxygen, pH, certain concentrations of carbon-nitrogen organics and others. The growth of Leptomitus lacteus resulted from the cooperation of these factors.