Cylindrospermopsis raciborskii,thought to be an originally tropical species, is found to become dominant and even formed blooms in many reservoirs of Guangdong Province. The dominance of this species presents a new threat to the region but little is known about its bloom forming mechanism. With the above reasons, the study aims to determine the factors that may lead to the successful dominance and bloom formation of C. raciborskii. The C. raciborskii filamental length, phytoplankton species composition, and different environmental physico-chemical parameters were examined monthly during the period of November 2014 and October 2015 in Zhenhai Reservoir. Our results showed that C. raciborskii dominated absolutely in the reservoir during the sampling period. Its biomass ranged from 5.9 to 15.5 mg/L, with a mean value of 11.3 mg/L, accounting for nearly 93.5% of the total phytoplankton biomass. C. raciborskii biomass was relatively high from February to June in 2015. The highest levels were recorded in June and the lowest in both October and November. Filamental length of C. raciborskii exhibited an obvious seasonal regularity. Correlation analysis shows a highly significant negative correlation between filamental length of C. raciborski and water temperature. The biomass of C. raciborskii was significant-positively correlated with total nitrogen and total phosphorus, and significant-negatively correlated with the ratio of nitrogen to phosphorus. Further analysis with continuously multiple factors analysis indicated that total phosphorus concentration was the key factor affecting C. raciborskii biomass. We speculate that the strong absorption to phosphorus and the high P-storage capacity of C. raciborskii have played an important role in the seasonal variation of biomass.