A modern design method, in which traditional design formulas are conjoined with numerical simulation and optimization, is successfully used to design the out-size extrusion frame precisely so that the press cost can be saved. A new technology used for decompressing by a multi-steps dynamical mode is put forward, which makes it possible to decompress the large flow-volume high-pressure oil in the main cylinders. In addition, a method for realizing the fixed mandrel process by hydraulic support is proposed and its control equation is established. Pre-tightening frame tests are carried out by over-operating pressure on 100 MN aluminium extrusion press with oil-driven double action, which is developed based on the above key techniques and is the largest press so far in the world, and the results show that the frame structure designed is reasonable and reliable, and the modem design method used is an useful tool for designing large and out-size heavy plastic forming machinery. The results of decompressing curve in main cylinder and noise inspection indicate that multi-steps dynamical mode for decompressing the large flow-volume high pressure oil is valid and reliable. Meanwhile, the fixed mandrel process is well realized based on the control equation. These key techniques have been used in the development of 125 MN aluminium extrusion press with oil-driven double action.