Calcite cement is widely distributed in the tight sandstone of Fuyu oil reservoir in Sanzhao Sag，and calcite cementation is an important controlling factor for reservoir densification. In this paper，taking calcite cement as the research object，the formation time and origin of calcite were identified by thin section identification，cathodoluminescence，inclusion test，laser Raman spectroscopy and scanning electron microscopy. The results show that the main types of calcite cements in Fuyu oil reservoir are fine-grained calcite and coarse-grained calcite. The fine-grained calcite fills intergranular pores and grows into crystal stock calcite，or replaces the tiny crystals of feldspar and precipitates in the dissolution pore or micropore in the surface of feldspar，which sometimes can cement many particles and grows into mosaic calcite；the coarse grained calcite is characterized by residual pore filling and distributes irregularly. The calcite cement developed in two periods. The first period is in Nenjiang Period（78-77 Ma years ago），and the formation temperature is 89.5-110.2 ℃；the second period is in Mingshui Period（73-65 Ma years ago），and the formation temperature is 122.6-134.3 ℃. The formation of early calcite is related to biogenic gas，and the formation of late calcite is related to the decarboxylation of organic acid.The formation of calcite cement in the two periods greatly destroys the reservoir physical property，which is the important reason for reservoir densification.