The lacustrine dolostone of Nenjiang Formation in Well CCSD-SK was taken as the study object in this paper. Well CCSD-SK is located in Qijia-Gulong depression of Songliao Basin, northeastern China, with its administration district belonging to Daqing City, Heilongjiang Province. Well CCSD-SK is the first scientific continental drilling well which mainly includes Cretaceous strata and is composed of double drilling holes located respectively in the south and the north. Lacustrine dolostone of Upper Cretaceous Nenjiang Formation has aroused much interest among geologists because it is not only a special terrane but also one of the most important oil-producing layers in Songliao Basin. Therefore, samples for this study were mainly collected from 1st Member of Nenjiang Formation in the south drilling hole. Lacustrine dolostone of Nenjing Formation has its special formation positions and shapes (mainly in layered form and subordinately in elliptic form), and layers of dolostone are interbedded with layers of mud rock, with oil shale at the bottom. Lots of Ostracoda and spinicaudatans are existent in dolostone, and a small amount of pyrite occurs in the mud rock. All this suggests a reduction environment with flourishing living things. It is thought that the formation of dolomite probably had an active effect on oil accumulation. The study of the mineralogical characteristics and the formation mechanism of lacastrine dolostone in Nenjiang Formation is hence of great significance in sedimentology. Using XRD and EPMA methods, the authors analyzed and studied mineralogical characteristics of dolostone samples. XRD results show that dolomite is the main mineral phase, quartz possesses the second position, and other minerals are small amounts of illite-smectite mixed layer, kaolinite, plagioclase, pyrite and calcite. EPMA shows that the main chemical compositions are CaO and MgO, which make up 39.778%～50.429%, and FeO and SiO2 possess the second position. Combined with XRD phase analysis, it is held that SiO2 is derived from quartz, whereas CaO, MgO and FeO come from the main mineral dolomite. The main mineral in the samples should be named ferruginous dolomite rather than ankerite because the content of MgO is much higher than FeO. This naming is consistent with the strict definition of mineralogy. The authors consider that the main mineral is ferruginous dolomite belonging to the transitional species between dolomite and ankerite rather than ankerite considered by previous researchers. The XRD results of refined parameters for the crystal unit cell also show that the samples are of the rhomb-centered hexagonal crystal having space group of R3, the parameters of a, c, V values are larger than those of the standard dolomite and much closer to those of the standard ankerite, implying that the samples and the standard ankerite have the same crystal structure; however, there exists difference in the amount of Fe2+ and Mg2+ at the same structural position, and the value arising of axis and volume results from Fe2+ substitution of part of Mg2+ in the crystal structure. It is thus thought that ferruginous dolomite is a transitional species between dolomite and ankerite in the light of crystal structure. The authors also calculated the values of the ordered degree and the mole content of CaCO3 on the basis of XRD data, and reached the conclusion that the samples having characteristics of relatively high calcium content (55.48 g/mol on average) and low ordered degree (0.40 on average) were formed in an unstable diagenetic environment characterized by rapid crystallization.