Water vapor flux over a planted coniferous forest ecosystem near Qianyanzhou, Jiangxi Province was continuously measured with the eddy covariance technique for the year of 2004. How environmental variables, including net radiation, air temperature, soil moisture affect water vapor flux variation was studied in detail. Results showed that the maximum monthly water vapor flux occurred in July, and the maximum daily water vapor flux reached 0.149 g m^-2•s^-1. While January has the lowest monthly water vapor flux value, the maximum daily water vapor flux in January was only 0.021 g m^－２•s^－１. The diurnal variation of water vapor flux showed different pattern for clear and cloudy days. Except in July and in October, water vapor flux for all other months was less than the precipitation. Although the monthly precipitation in October was only 0.6 mm, water vapor flux didn’t show a significant decrease as compared with previous years. The annual total evapotranspiration was 736.1mm, which was about 55.6% of the annual precipitation (1323.6 mm). Regression analysis showed that daily water vapor flux was significantly correlated with net radiation, air temperature, soil temperature and soil heat flux under both clear and cloudy days, all in quadratic relationships. Partial linear regression analysis indicated that on clear days relative humidity and air vapor pressure deficit have a relatively larger impact on water vapor flux than other variables. While on cloudy days relative humidity and soil moisture content have a larger impact on water vapor flux, even though not in a significant level. We didn’t find a significant relationship between wind speed and water vapor flux. Stepwise regression analysis demonstrated that different set of environmental factors controlled water vapor flux on days with different weather conditions. The most important environmental factors controlling water vapor flux were net radiation, air and soil temperature, and air vapor pressure deficit for clear days, and net radiation, soil temperature relative humidity，soil heat flux and soil moisture content for cloudy days. From this study, it was clear that these environmental variables, especially net radiation and soil temperature regulated water vapor flux over the planted coniferous ecosystem.