The invention discloses an adjustable pressurization device of an assembly type steel support of a foundation pit and an application method of the adjustable pressurization device, and relates to thetechnical field of foundation pit supporting construction. A first steel support device and a second steel support device are well connected in an insertion mode and then fixed to connecting beams correspondingly; pressurization devices are started and conduct pressurization on the connecting beams, prestress is applied to a steel support system, the connecting beams generate displacement along with the prestress and drive the first steel support device and the second steel support device to be separated in the opposite directions at the same time, and therefore a top open space formed betweenthe first steel support device and the second steel support device can be enlarged; and early-strength and high-strength concrete is poured into the space to form a concrete supporting block, and thepressurization devices are disassembled after the strength of the concrete supporting block is equal to the designed strength. The pressurization devices are disassembled, the loss of the axial prestress basically does not exist, the accuracy of prestress application is guaranteed, and deformation of the foundation pit is more controllable. In order to achieve better cooperation of the first steel support device and the second steel support device and convenient installation, locating ribs are arranged on the outer side walls of first side plates, and locating grooves which are matched with the locating ribs are formed in the inner side walls of second side plates.