Abstract: Aiming at studying the effects of auto-toxicity substance m-hydroxy phthalic acid(SA) on seedlings of Cunninghamia lanceolata under osmotic stress,the experiments were conducted by aquaculture of 1 year old seedlings of C.lanceolata simulantly treated by PEG-6000 with gradient concentrations(50,100,200 mg/L) to study the influence of different SA concentrations upon the chlorophyll content of the seedlings.The results showed that the chlorophyll content of the seedlings presented a declining trend along with the increment of the drought stress caused gradient concentration of PEG-6000 from -0.3 MPa,-0.5 MPa to -0.7 MPa,The higher drought stress level was,the faster chlorophyll content decreased.In comparison with CK,while PEG-6000 concentration reached 200 mg/L,the chlorophyll content deceased by 15.39%,25.78%,and 41.89% respectively on the 7 th,14 th,and 21 st day of the experiments.