The primary factor quantitative reservoir assessment method was used in this paper to precisely evaluate the clastic reservoir property of the Shahejie Formation in Dongying Sag. The reservoir capacity parameters (porosity and permeability), grain sorting, matrix content, granularity, extent of being buried and pressed, solution and cementation degree were chosen as the reservoir quality assessment parameters. Weight coefficients of other factors related to porosity which is the primary factor are obtained by grey correlation analysis. Then, the reservoir assessment classification is determined by the comprehensive reservoir quality assessment gene “Q” . When there are short of porosity and permeability data, the relevant weight coefficients of other factors are obtained by their correlations with porosity as well, and the comprehensive reservoir quality assessment gene Q1 is obtained by the similar method as Q. The result of reservoir assessment of the samples given by  “Q” and Q1 is consistent with that by the clastic reservoir classification criterion which was offered by CNPC. The method was used to classify and evaluate the Shahejie Formation clastic reservoir and the result was compared with the oil testing result, the two results were highly consistent with each other. The analyses show that this method is practical in reservoir comprehensive assessment. According to the value of Q or Q1, the reservoir quality of some interval in some area can be quickly predicted.