By using the drilling coring data, rock thin section, cast thin section, scanning electron microscope and logging data, and combined with the analysis of the test data, the diagenesis characteristics and its impact on reservoir porosity of Lenghu region of Paleogene has been carried on by the detailed study. The results showed that the maturity of the Lenghu region is low, the primary pore, secondary pore and micro cracks are both developed in the reservoir. The diagenesis types are complex, including compaction, cementation and dissolution. The compaction is the most fundamental reason for failure of reservoir. The porosity loss of the reservoirs in the study area caused of the compaction is 21.65 % in average. The cementation is an important factor for further reducing the reservoir pore loss, causing an average of 3.34 %. The dissolution of the reservoir is an important way to increase the hole, but due to the lack of dissolution fluid, its contribution is limited, increasing pores about 2.26 %. There is a positive correlation between the reservoir properties and maturity, and a negative correlation between the properties and depth, which show that the low maturity and deep buried Lulehe formation (E₁₊₂) has the strongest compaction and cementation, and the worst reservoir property. The property of lower Ganchaigou formation (E₃ ¹) was medium, and the upper of lower Ganchaigou formation(E₃ ²), which has the highest maturity and is the most shallow buried has the optimal reservoir property.