Abstract: Based on the analysis of core, rock thin section, cast thin section, SEM and X-ray diffraction data, it is considered that the diagenesis of the lower Wuerhe formation of Middle Permian in the eastern Mahu sag includes compaction, cementation and dissolution, among which compaction and cementation resulted in poorer petrophysical properties, and the dissolution, especially the dissolution of feldspar and laumontite allowed the improvement of the reservoir physical properties. The comprehensive analysis of type and evolution of the diagenesis shows that the diagenetic evolution of the lower Wuerhe formation is mainly at the Stage A of middle diagenesis period in the study area, partial reservoirs with relatively large burial depth are at Stage B of middle diagenesis period or at Stage A of late diagenesis period. According to diagenesis, cement content and pore type, the lower Wuerhe formation is classified into 6 diagenetic facies such as weak compaction diagenetic facies of mud-poor medium-coarse sandstone—fine conglomerate, dissolution diagenetic facies of analcite—laumontite?cemented medium to coarse sandstone—conglomerate, calcite?cemented diagenetic facies of shaly sandy conglomerate, weak?dissolution diagenetic facies of analcite—laumontite cemented sandy conglomerate, silicon—albite cemented diagenetic facies of dolomitic sandy conglomerate and heulandite?cemented diagenetic facies of mud?rich sandy conglomerate, among which the weak compaction diagenetic facies of mud-poor medium-coarse sandstone—fine conglomerate is the most favorable, dissolution diagenetic facies of analcite—laumontite cemented medium to coarse sandstone—conglomerate is secondly favorable and the poorest facies is heulandite?cemented diagenetic facies of mud?rich sandy conglomerate