Based on the rock thin section observation, cast image analysis, scanning electron microscope and capillary pressure curves, pore genetic type and pore structure of Chang 6 reservoir in Yongjin area, Zhidan Oilfield are analyzed and studied. The result shows that Chang 6 reservoir is characterized by low porosity and ultra-low permeability. The pore types mainly include primary intergranular pore, secondary intergranular dissolved pore and intragranular dissolved pore. Feldspar corrosion is the main cause of the forming of secondary pore. Middle-small pore and fine-micro throat are the main pore throat characteristics, which provides the major reservoir space and seepage channel of Chang 6 reservoir. The pore structure is divided into three types. Middle-small pore and fine throat （type Ⅰ） and small pore and micro throat （type Ⅱ） are considered as the dominated reservoirs of Chang 6, and micro pore and micro throat （type Ⅲ ） is non-effective reservoir.