The invention discloses a construction method of a deep foundation pit open caisson self-sinking support system. The construction method comprises the following steps that after the pay-off, blade feet made by the cast-in-place are measured, spray heads are pre-buried in the blade feet, a first section ring beam is subjected to cast-in-situ manufacturing, a first section prefabricated open caissonwell body is installed, steel pipes are pre-buried in the open caisson well body, after the open caisson well body is connected with the blade feet, the soil body is cut by water injection and mud isdischarged, and the first section open caisson well body sinks to the designed elevation, after the ring beam is subjected to cast-in-place construction, the next section open caisson well body is installed, the water injection and sinking are carried out by adopting the same method as the first section open caisson well body, all the open caisson well bodies are installed in sequence, so that the open caisson well bodies of all the sections sink in place, and bottom sealing and mud drainage are carried out. According to the construction method of the deep foundation pit open caisson self-sinking support system, the open caissons are assembled in a prefabricated manner, the soil body is converted into the mud to be discharged outwards to enable the open caissons to sink automatically, thesoil pressure and the bottom water pressure at the peripheries are balanced by utilizing the mud in the open caisson, the excavation difficulty is reduced, and the stability in the sinking and excavation process is ensured.