The previous studies on the conjugation water depth and the length of free hydraulic jump for a gradually expanding stilling basin were analyzed. The height of obstacle at the end of a gradual expanding flat-bottom stilling basin was calculated by different calculation methods. The comparative analysis of calculation results shows that the method in the literature ［1］ is more reliable, and the height of obstacle calculated by the method in the literature ［7］is too small. Finally, the paper analyzed the factor causing the calculation value of obstacle height being too small，and suggested the calculation method in literature ［1］ should be used.