There is a new type of diagenesis trap of conglomerate body which is sealed by root fan in northern Dongying sag. Based on core observation, grain size analysis, thin sections identification and clay mineral analysis, the lithofacies architecture and different diagenetic evolution of root fan and middle fan are studied, the genetic mechanism and controlling factors are proposed. The root fan lithology is conglomerate with complex matrix, the content of matrix is high, which disfavors the preservation of primary pores with the depth of burial. The main diagenetic types include the compaction and the recrystallization, and the porosity and permeability are persistently declined. The middle fan lithology is grain-supported pebbled sandstone or conglomeratic sandstone, the content of matrix is lower, which favors the preservation of primary pores. The main diagenetic types include the compaction, cementation,dissolution and replacement in acid environment, which formed several secondary pore zones. So, the lithofacies architecture is the foundation for the diagenesis trap, and the difference of diagenesis between root fan and middle fan is determinant for the diagenesis trap. The influences of diagenesis trap include sedimentary sequence, lithofacies architecture and diagenesis fluid.