The deep fan diagenetic trap developed widely in the northern steep slope zone of the Dongying sag and the differences in sedimentary fabric and diagenesis play important role in the reservoir forming. How to evaluate the trap effectiveness is the key to exploration. By analyzing geological characteristics of root-fan and oil reservoir，the object of trap evaluation，the root-fan sealing mechanism and evaluation principle were determined to predict oil-bearing properties of diagenesis trap effectively. Based on this，according to different data at different exploration stages，the trap effectiveness evaluation methods suitable for different exploration stages were established. The results show that the root-fan developed high clay heterobase and strong compaction and heterobase recrystallization so the root-fan could be a lateral sealing seal or a local seal. The main seal mechanism of diagenetic trap was capillary sealing，the difference of displacement pressure between root-fan and mid-fan determined oil column height. Mercury injection data should be corrected by experimental data of oil displacing water in whole diameter core，and the method of calculating breakthrough radius of oil displacing water using NMR data or routine physical data were established，by which the breakthrough pressure difference under oil reservoir condition can be obtained to evaluate the trap effectiveness. The analysis of the applied result shows that this evaluation method can be used to guide exploration.