Xinggu buried hill reservoir is a one with giant thick metamorphic rock. It has the characteristics of strong reservoir heterogeneity，developed fractures，up to 2 300 m of oil column，development through natural energy and rapid decline of oil production. It is difficult to optimize reasonable development mode for insufficient formation energy supply by water flooding. In view of dual media characteristics of the oil reservoir，the experiment process was improved，and an experiment method of soaking under various pressures for the dual media reservoir was put forward. Oil displacement by osmosis in matrix was quantified，and oil displacement mechanism of gas injection flooding was studied thoroughly according to numerical simulation research and field test analysis. The research result shows that gas injection flooding in the giant thick metamorphic rock buried hill reservoir can play a role in gravitationally flooding，gas floatation oil displacement and matrix osmosis，and greatly improve the sweep volume，which can act as a guidance for three-dimensional gas injection development design in Xinggu buried hill reservoir. The gas was mainly injected at the top with supplementary injection at the middlelow part. The role of gas floatation oil displacement can be played during gravity drive，and oil recovery efficiency has been enhanced by 15% compared with that of natural energy production.