There are abundant primary sedimentary structures in the Middle Triassic Flysch , which is up to 5000 m in thickness, in the Nanpanjiang Basin in southwestern China. A new type of sedimentary structures, syn-sedimentary compression structures, is found and distinguished from these structures. The syn-sedimentary compression structures include compression wrinkles, compression sand pillows, compression fractures and compression ridges, which are found on the surfaces of mud units or the bottoms of sand units of the flysch deposits. The formation processes of all these compression structures are qualitatively simulated with dough, and their origin is discussed. All these structures were formed after the deposition and before the diagenesis, recording compression stresses in the sedimentary basin in the Middle Triassic. The orientations of the compression structures indicate that the flysch basin contracted in a SSW-NNE compression during the Middle Triassic. Thus, the syn-sedimentary compression structures can provide new evidences and constraints on the dynamic evolution of flysch basins.