With global warming, the influence of water vapor, which is the most abundant greenhouse gas in atmosphere, on greenhouse effect will be further enhanced. Therefore, the greenhouse effect of tropospheric water vapor was analyzed by line by line model. The results show that there are fundamental differences of the contributions of tropospheric water vapor to the "warm earth" before Industrial Revolution and the current "global warming". The warm environment before Industrial Revolution is primarily due to the absorption of the earth emission by the water vapor below 3 km, which amounts to 50% of the total absorption. However, the strong absorption band of the water vapor adjacent to surface has almost saturated, consequently, the increase of water vapor in upper troposphere contributes most to the current global warming.