Abstract: An atomic parameter （gi） for non-hydrogen atom i is defined as:ｇｉ＝（ｎｉ－１）０．５·ｍｉ·（（ｔｃ）/（ｔｉ））αｉ－（－１）ｈｉ＋ｋｉ（１＋ｋ２ｉ）ｈβｉｉ.A novel topological index (mL) is built up with（gi）in this paper.The quantitative structure-activity relationships(QSAR) studies are performed in relation to the acute toxicitiea and 0L of substituted aromatic compounds (with substituents —Cl,—OH,—NO2  and alkyl).The linear regression models between the 96h-LC50 and 0L of the compounds to aquatic organisms are proposed as follows:(1) sword fist  -lg　LC50=-0.949　9+0.0306　0L,R=0.977　4；(2) rare minnow  -lg　LC50′=-1.165　8+0.035　40L，R=0.995　4.The values calculated by the models are basically true as the experiment values.

Key words: quantitative structure-activity relationship, substituted aromatic, acute toxicity, sword fish, rare minnow