The evaporites in the lower part of the Jiufotang Formation of Naiman Sag，Kailu Basin, were widely developed, forming the world’s second-largest trona deposits, which serve as important deep-time records of the Early Cretaceous palaeoclimate and palaeoenvironment in Northeast Asia. However, the genesis of the trona deposit and its source-to-sink processes are still unclear. In order to investigate the formation processes of soda ash, we carried out a multi-scaled investigation of the sedimentary route system, involving regional “basin-mountain coupling”, core-scaled sedimentary cyclothemes and thin section-scaled analysis of mineral assemblages. And the material sources, facies distribution, key interfaces and sedimentary differentiation mechanisms of the Naiman trona deposit were explored. The results show that the Naiman trona deposit is similar to the Green River Formation in the United States, and the evaporites were precipitated in the central depression where black shales were also widely developed. Evaporite facies conform approximately to a bull’s eye pattern with a zonation of terrigenous clasts, alkaline earth carbonate, surrounding a basin centre accumulation of Na-carbonate and halite, indicating that the depositional differentiation is controlled by the solubility and geomorphic elevation of the basin. The formation of Naiman trona deposits mainly involves two processes: firstly, the Na⁺-HCO{}_{\mathrm{3}}^{-} rich brine accumulated in the lake during the humid period with large amounts of Na⁺, Ca²⁺, Mg²⁺, HCO{}_{\mathrm{3}}^{-}, Cl^- leached from the intermediate-basic volcanic rocks around the periphery of the basin. Secondly, the lake basin was highly dried under extreme arid climate conditions, and the lake was saturated with trona-halite, which were precipitated sequentially in the center of the depression. The giant trona deposits are characterized by interbedded halite, trona, gypsum, and black shale, reflecting frequent fluctuations of the lake level and cyclicity of trona accumulation. The annual varves and Milankovitch cycle of evaporites in Naiman Sag indicate that the trona-accumulating period was about 1.2 Ma, much smaller than the depositional time of the lower part of the Jiufotang Formation (2.5 Ma). The source-to-sink depositional pattern of trona deposits in the Naiman Sag will provide a reference for the exploration and development of similar Na-carbonate evaporite deposits.