Backflow exists in the inverted U-tube steam generator (UTSG) under the condition of natural circulation, which affects the heat carrying capacity and natural circulation capacity of the primary circuit coolant system. Referring to the design parameters of UTSG in PWR thermal experimental device (PWR PACTEL) in Finland, this paper uses computational fluid dynamics (CFD) software Fluent to simulate the backflow in UTSG under the condition of uniform flow decline, and studies the influence of primary side operation parameters, UTSG design paramet...