For the extension length of the short stiffener at the outer wall’s support of the rectangular caisson，we take a gravity caisson pier project as an example and calculate the actual stress of the caisson members using the finite element software，according to which the extension length of the short stiffener of the support is analyzed. The calculation results show that，for some cases，the length of the short stiffener extending out of the support is greater than 1/4 of the clear span of the slab. Therefore，for the extension length of the short stiffener of the rectangular caisson outer wall’s support，the minimum length specified in the code can not be simply taken. It should be determined by further calculation.