Abstract: Using seven species of plants as materials, after exposing them to SO2, the author found that K+ efflux in plant leaves is independent of the concentration of K+ in the culture solution and the leaves. The efflux of K+ changes with different species, and the intensity of light, wider opening of the stoma and high sulphur content in the leaves make larger K+ efflux. They are closely linked with one another. On the basis of this pattern, the author evalutes the pollution situation of Shengyang by dividing it into four classes in accordance with the threshold and statistic methods (X±S) of the K+ efflux in Robinia pseudoacacia, and the result is the same as that by using the physico-chemical monitering method. The relevent equation between the SO2 concentration and the K+ efflux is as follows: Ŷ=0.0111X-0.0259(r=0.879, P<0.01). In accordance the SO2 concentration in the atmosphere was calculated from this equation in 1982 and 1988 and the forecast pictures were 95% true. This result shows that the new method is more reliable than the physico- chemical one.