The population dynamics of Bactrocera dorsalis (Hendel) was monitored with pheromone methyl eugenol traps all year around from 2002 to 2005 in Guangzhou, Guangdong province. Investigation showed that B. dorsalis occurs all year round in Guangzhou. The population increased from May rapidly. The population decreased gradually from October. Afterwards, the population remained low from November to March of the next year. The climate factors which influence the population fluctuation of B. dorsalis were analyzed by principal component analysis and correlation analysis. The result showed that the temperature-rainfall factor is the main influence, and monthly mean rainfall is the key factor which influence the population fluctuation of B. dorsalis; the sunlight hours factor is the secondary influence, but it has no obvious relevance with the monthly mean amounts of B. dorsalis.