Abstract: Under the natural circulation of the primary loop, the backflow may occur in the partial U-tubes of an inverted U-tube steam generator (UTSG), which brings negative effect on natural circulation. The relationship between the critical pressure drop and the tube length of the inverted U-tube is deduced by theoretically analyzing the hydrodynamic curve of the inverted U-tube of a steam generator in a UTSG, and the numerical analysis is performed using the system analysis program RELAP5. It is shown that the critical pressure drop decreases first and then increases when the pipe length increases, that is, the shortest inverted U-shaped tube of the small UTSG steam generator first reverses, while the longest tube of the large UTSG steam generator first reverses. The conclusions explain the phenomenon of different distribution of the reflux tubes obtained by different simulations, and provide a theoretical basis for the prediction of the spatial distribution of the reflux tubes in the inverted U-tube of the UTSG.