Abstract: In spite of rapid progress achieved in research work of the assessment of environmental quality,the problem of weighting of various factors has never been really solved so far.Strictly speaking,weighting should be based upon the objective sub-contribution of specific assessment parameters to the total environment and their biological effects.But so far as recent study case in our country and abroad,due to quite a few reasons,the conventional practice of weighting has still followed the way of subjective judgement with strong artificial influence.This paper presents a new approach of weighting,i.e.P_j factor analysis,based upon an attempt of comprehensive assessment of environmental quality in Baoshan-Wusong District of Shanghai.Our approach revised the conventional weighting practice on two direction.First,measuring environmental value P_j as an original information of weighting in same a standard was used stead of subjective judgement.Secondly,it introduced the approach of factor analysis to estimate the objective sub-contribution of specifitc assessment parameters and their interrelationship.Thus making possible to trace and interprete pollutants distribution,migration and reorganization in a regional system.Especially today,as the means of environmental monitoring has been electronized, the weighting method of P_j factor may be used as a prerequisite for overall modernization of environmental quality assessment.