The invention belongs to the field of machine manufacturing, particularly relates to a heavy adjustable rotary mechanical arm for pressing and a method, and provides the following scheme aiming at the problem of insufficient means in the prior art: the heavy adjustable rotary mechanical arm comprises a base, a main arm, a support arm and a lock mechanism, the base comprises a support, a rotating shaft sleeve with an oil groove and a nut with a shoulder, the base is connected with one end of the main arm through a rotating shaft sleeve with an oil groove, the other end of the main arm is provided with a U-shaped opening, one end of the supporting arm and the lock mechanism are wrapped in the U-shaped opening of the main arm, the design of brand new technological equipment embodies the mechanical/non-mechanical interaction requirement, switching between a pneumatic locking mode and a mechanical locking mode is provided for the mechanical arm in the working state, and the mechanical arm is convenient to use. And the failure risk after the mechanical arm presses the workpiece is eliminated, and the manual operation risk in the workpiece machining process is reduced. The process equipment is obvious in effect after being used and verified in a workshop, and the manufacturing requirements of products are met.