The shallow reservoirs of Youquanzi oil field in the northwest Qaidam basin are characteristic of abnormal low pressure.Based on the study of low pressure distribution, the main controlling factors of low-pressure reservoirs formation in the Youquanzi were analyzed including fluid features, seal mechanism, structural evolution and sedimentary characteristics. The seal conditions, formation mechanism and the relationship between pressure evolution and hydrocarbon accumulation in the shallow reservoirs were discussed.The study results show that: the formation water type is CaCl2 with high salinity and low ratio of Na+ and Cl- , which reflects a good sealing capability; additionally, the light density and low viscosity of crude oil also reflect good sealing conditions of the shallow reservoirs. The plastic gypsum mudstone deposited in saline lacustrine, the bitumen plug, the thrust fault plane and the reservoir heterogeneity provide both vertical and lateral sealing conditions. Methods of EASY%Ro and stratigraphic correlation were used to estimate the late Neogene denudation thickness of Youquanzi with a result of 2 700 m. Influenced by the intense tectonic uplift, the pore rebounded and the temperature decreased, which lead to a pressure drop of 14.1 MPa and 7.4 MPa respectively. The abnormal low pressure of the shallow reservoirs in Youquanzi oilfield is controlled by multi effects of sedimentary assemblage, tectonic evolution and hydrocarbon accumulation.