Large open caisson construction method in soft geological shaft

The invention discloses a large open caisson construction method in a soft geological shaft. The large open caisson construction method comprises the following steps of: firstly conducting preliminary flattening to an excavated surge shaft platform, then conducting formwork erection and casting of a first open caisson section, starting excavation sinking construction of the first open caisson section after the first open caisson section is cast, stopping excavation construction when the top of the first open caisson section is sunken to the surge shaft platform or a position which is slightly higher than the surge shaft platform, conducting formwork erection and casting of a second open caisson section, then conducting excavation sinking construction of the second open caisson section, and conducting construction according to the alternate operation method of formwork erection and casting and excavation sinking construction of the open caisson till design depth is reached. By adopting the sinking construction of the open caisson and by adopting a hard-first and soft-later excavation method during excavation, not only can the safety be guaranteed, but also the smooth sinking construction can be guaranteed and concrete defects caused by inclination and sudden falling are prevented. The large open caisson construction method additionally has the advantages that the arrangement is reasonable, the overall planning and all-round consideration are made, the construction period is effectively shortened, the investment is reduced, and the like.