Unconventional volcanic reservoir is different from conventional reservoir in reservoir space, diagenesis, pore formation and evolution. The Carboniferous volcanic reservoir was selected in Junggar Basin, Northwest China because based on sediment/rock cores and outcrop data, diagenesis and pore evolution were studied by elemental measurements, thin section observations, and diagenetic analyses. These analyses shows that the reservoir lithology is predominantly intermediate-basic volcanic, and the reservoir storage space is composed mainly of secondary dissolved pores and fractures. The reservoir displays great heterogeneity, and has experienced a great variety of diagenetic alteration during various diagenetic stages including: (1) eruption fragmentation, crystallization differentiation and condensing consolidation at consolidated diagenetic stage; (2) metasomatic alteration, filling, weathering and leaching, dissolution by formation fluids and tectonism at the epigenetic modifications stage. The formation and evolutionary process of the pores is extremely complicated. The primary pores were formed during the consolidated diagenetic stage, and laid a foundation for the late development and alteration of effective reservoir. During the epigenetic modifications stage secondary reservoir storage space was developed via the formation of secondary pores and the development of fractures through weathering and leaching, dissolution by formation fluids and tectonism.