The origin of extra low-permeability and influential factors of Chang6 reservoir in the Yan25 area of Ansai Oil Field are studied synthetically through using the log information, testing and analytical information such as Casting section, scanning electron microscope, X diffraction, grain size analysis and mercury penetration et al. and then, the reservoir is evaluated. The results show that accumulation properties are worse, the reservoir mainly develops intergranular pore and emposieu, and pore structure types are fine pore and microscopic throat. Sedimentation and diagenesis have impacted on the origins of extra low-permeability together, such as accumulation properties are various in the different Sedimentary microfacies. The great content of low-energy component, fine grade and the low compositional maturity are the main reasons that cause the extra low-permeability. Compaction and cementation have decreased the porosity, then the permeability have become worse. However, denudation and fractures have improved the extra low-permeability characteristics to some great extent. According to the analysis of properties and pore structure parameters, the reservoir is classified into 3 classes, subsea river channel and mouth bar sand body are the main distributed facies belt of the reservoir.