The pore structure of rocks significantly influences the porosity and permeability of reservoirs and the migration ability of oil and gas, and being the key task on the development of volcanic gas reservoirs. Nine volcanic rock samples from the Yingcheng Formation and Huoshiling Formation in the Longfengshan area of the Changling Fault Depression in the Songliao Basin were selected for this study. The pore structures of the volcanic rocks in the study area were investigated using high-pressure mercury injection, X-ray diffraction combined with fractal theory. The relationships between the fractal dimension and physical properties characteristics, pore structure parameters, and mineral content were analyzed to provide guidance for the development of volcanic rock gas reservoirs. The results show that the reservoir can be divided into 3 types (I, II, and III) based on the shape of the capillary pressure curve, and the physical properties deteriorate successively. Different types of reservoirs exhibit different fractal characteristics. For types I, II and III, the average total fractal dimensions were 2.3418, 2.6850, and 2.9203, respectively. The larger the fractal dimension, the stronger the heterogeneity of reservoir. A small number of macro-pores primarily contributed to permeability. The fractal dimension was negatively correlated with porosity and permeability. The fractal dimension of the rock was strongly correlated with quartz and feldspar contents, and the mineral composition and content are closely related to the pore evolution of the reservoir, which are the internal factors affecting the fractal dimension of volcanic rock.