The four major Chinese carps are commercially important fish species with high production in China. However, their recruitment decreased sharply in the Yangtze River since the late 20^th century. In the present study, to reveal the relationships between spawning activities of the four species and environmental factors, drifting eggs were collected at Yidu City, in the middle reaches of the Yangtze River, from May to July each year between 2005 and 2010. Classification and regression trees (CART) analysis was applied to identify the key factors associated with spawning activities of the four carp species. Twelve predictor variables (hydrological and meteorological variables) and one response variable (egg presence or number of egg) were included in the CART. Our CART analysis showed that water temperature and the diurnal increase of water level were the two most significant factors for the spawning activities. When water temperature was between 18°C and 24°C, especially in association with the diurnal increase of water level greater than 0.55 m·d^-1, spawning activities was always favored. Unlike the hydrological factors, meteorological factors seemed to have no influence on initiating the spawning activities. The density of drifting eggs of the four species was mainly influenced by the diurnal variation of water level, the diurnal variation of water discharge, water temperature, humidity, and air pressure. We then related our results to the ecological regulation of the Three Gorge Reservoir in the middle reaches of the Yangtze River. We suggested that, when water temperature was between 18–24°C from May to July, to ensure the successful spawning of the four carp species, the ecological regulation should be managed to create flood peaks and make the diurnal increase of water level greater than 0.55 m·d^-1.