The sandstone and mudstone ores from the Upper Cretaceous Qingshankou Formation in DL uranium mineralized belt of Qianjiadian Sag, southwestern Songliao Basin were investigated to understand the uranium occurrence, enrichment mechanism and metallogenic process. This research provided important guidance for further uranium prospecting and in-situ leaching both in this basin and other similar area.Methods: The rock and mineral identification, SEM observation, energy spectrum and electron probe analysis, whole rock U-Pb isotopic dating have been carried out in this paper. Results:The ores with the low maturity of the composition and structure, were deposited in braided fluvial that have multiple sources. Uranium in the all ores were composed of pitchblende, followed by some uranium bearing titanium minerals, and a little uranium bearing carbonate minerals, the mixtures bearing uranium, zirconium and silicon, and adsorbed uranium substance. The pitchblende, characterized by colloidal, lumpy and particulate, mainly occurred within the local sorptive and reductive position, such as the matrix, dissolution pores or edges of various clastic minerals. Also, they are closely associated with carbonaceous organic matter, pyrite, kaolinite, montmorillonite, ankerite and titanium bearing minerals. A number of isochron ages of 50.6±1.6 Ma, 32.2±3.9 Ma, 27±4 Ma, 26.0±2.7 Ma and 23.9±2.8 Ma were obtained, indicating the main metallogenic events took place during Paleogene. Conclusions:Two kind fluids including weak acidic reducing fluid and alkaline reducing hydrothermal fluid were contributed to the uranium enrichment and mineralization within the target layer. On the basis of tectonic-sedimentary evolution process, the four stages multi-fluids coupling and superimposed uranium metallogenic model for the Upper Cretaceous Qingshankou Formation of DL uranium mineralized belt in Qianjiadian Sag, southwestern Songliao Basin are preliminarily constructed: ① uranium preconcentration stage during sedimentary and diagenetic era; ② initial metallogenic stage by tectonic inversion since the end of Nenjiang era; ③ main metallogenic stage of hydrothermal fluid during Paleogene; ④ superimposed uranium enrichment stage during Neogene.