Abstract: Air pollution is one of the most important environmental problems. Planting trees with strong pollutant_absorption and pollutant_resistant ability in the city is an effectively supplementary way to alleviate the pollution. Studies on the po llutant_resistant ability of trees were limited to some simple ways as follows: observing leaf damage symptoms with eyes or doing fumigation experiment on plants with different densities of air pollutants. Due to the lack of standard methods to judge pollution_resistant ability of plants, wrong conclusion could be dr awn that the same plant species has different degrees of pollution_resistant abilities based on varied methods. In this paper, the Air Pollution Tolerance Index method is recommended for judging the pollutant_resistant ability of plants, with the morphological, anatomical, ecological and physiological features of plant species, all taken into account. Pollutant absorption ability of plants will be ascertained according to the original method of comparing plants, pollutant content in the air pollution region with those in the control one. Those trees with both strong pollutant absorption ability and pollutant_resistant ability should be chosen for urban greening.