Abstract：According to a comprehensive analysis of the macroscopic, microscopic and geochemical characteristics of the Qixia Formation reservoir, the major constructive diagenesis has syndepositional periods of dissolution, buried dissolution, tectonic uplift dissolution, mixed water dolomitization, buried dolomitization and recrystallization, of which the buried dissolution and the buried dolomitization seem to have made the greatest contribution to the reservoir space in that the intercrystalline pores, intercrystalline dissolution pores, dissolved holes and slits in them constitute the main reservoir space. Recrystallization possesses the second importance. Recrystallization did not make much contribution to the reservoir space, but it enhanced the effective porosity and permeability of the rocks, thus provided conditions for later dissolution. Nevertheless, the syndepositional dissolution pores have almost completely disappeared due to subsequent pore compaction, cementation and other damages.